### Overview
This PR introduces the ability to avoid copying the content of string
object into replies (i.e. bulk string replies) and to allow I/O threads
refer directly to obj->ptr in writev iov.

### Key Changes
* Added capability to reply construction allowing to interleave regular
replies with copy avoid replies in client reply buffers
* Extended write-to-client handlers to support copy avoid replies
* Added copy avoidance of string bulk replies when copy avoidance
indicated by I/O threads
* Minor changes in cluster slots stats in order to support
`network-bytes-out` for copy avoid replies
* Copy avoidance is beneficial for performance despite object size only
starting certain number of threads. So it will be enabled only starting
certain number of threads. Internal configuration
``min-io-threads-copy-avoid`` introduced to manage this number of
threads

**Note**: When copy avoidance disabled content and handling of client
reply buffers remains as before this PR

### Implementation Details
####  ``client`` and  ``clientReplyBlock`` structs:
1. ``buf_encoded`` flag has been added to ``clientReplyBlock`` struct
and to ``client`` struct for static ``c->buf`` to indicate if reply
buffer is in copy avoidance mode (i.e. include headers and payloads) or
not (i.e. plain replies only).
2. ``io_last_written_buf``, ``io_last_written_bufpos``,
``io_last_written_data_len`` fields added ``client`` struct to to keep
track of write state between ``writevToClient`` invocations
####  Reply construction:
1. Original ```_addReplyToBuffer``` and ```_addReplyProtoToList``` have
been renamed to ```_addReplyPayloadToBuffer``` and
```_addReplyPayloadToList``` and extended to support different types of
payloads - regular replies and copy avoid replies.
3. New ```_addReplyToBuffer``` and ```_addReplyProtoToList``` calls now
```_addReplyPayloadToBuffer``` and ```_addReplyPayloadToList``` and used
for adding **regular** replies to client reply buffers.
4. Newly introduced ```_addBulkOffloadToBuffer``` and
```_addBulkOffloadToList``` are used for adding **copy avoid** replies
to client reply buffers.

####  Write-to-client infrastructure:
The ```writevToClient``` and ```_postWriteToClient``` has been
significantly changed to support copy avoidance capability.

####  Debug configuration:
1. ``min-io-threads-avoid-copy-reply`` - Minimum number of IO threads
for copy avoidance
2. ``min-string-size-avoid-copy-reply`` - Minimum bulk string size for
copy avoidance when IO threads disabled
3. ``min-string-size-avoid-copy-reply-threaded`` - Minimum bulk string
size for copy avoidance when IO threads enabled

### Testing
1. Existing unit and integration tests passed. Copy avoidance enabled on
tests with ``--io-threads`` flag
2. Added unit tests for copy avoidance functionality

### Performance Tests

Note: pay attention `io-threads 1` config means only main thread with no
additional io-threads, `io-threads 2` means main thread plus 1 I/O
thread, `io-threads 9` means main thread plus 8 I/O threads.

#### 512 byte object size
Tests are conducted on memory optimized instances using:
* 3,000,000 keys
* 512 bytes object size
* 1000 clients

|io-threads (including main thread)	|Plain Reply	|Copy Avoidance	|
|---	|---	|---	|
|7	|1,160,000	|1,160,000	|
|8	|1,150,000	|1,280,000	|
|9	|1,150,000	|1,330,000	|
|10	|N/A	|1,380,000	|
|11	|N/A	|1,420,000	|

#### Various object size, small number of threads

|iothreads |Data size |Keys |Clients |Instance type |Unstable branch
|Copy Avoidance On |
|---	|---	|---	|---	|---	|---	|---	|
|1	|512 byte	|3,000,000	|1,000	|memory optimized	|195,000	|195,000	|
|2	|512 byte	|3,000,000	|1,000	|memory optimized	|245,000	|245,000	|
|3	|512 byte	|3,000,000	|1,000	|memory optimized	|455,000	|459,000	|
|4	|512 byte	|3,000,000	|1,000	|memory optimized	|685,000	|685,000	|
|	|	|	|	|	|	|	|
|1	|1K	|3,000,000	|1,000	|memory optimized	|185,000	|185,000	|
|2	|1K	|3,000,000	|1,000	|memory optimized	|235,000	|235,000	|
|3	|1K	|3,000,000	|1,000	|memory optimized	|450,000	|450,000	|
|	|	|	|	|	|	|	|
|1	|4K	|1,000,000	|1,000	|network optimized	|182,000	|187,000	|
|2	|4K	|1,000,000	|1,000	|network optimized	|240,000	|238,000	|
|	|	|	|	|	|	|	|
|1	|16K	|1,000,000	|500	|network optimized	|100,000	|120,000	|
|2	|16K	|1,000,000	|500	|network optimized	|140,000	|140,000	|
|3	|16K	|1,000,000	|500	|network optimized	|275,000	|260,000	|
|	|	|	|	|	|	|	|
|1	|32K	|500,000	|500	|network optimized	|57,000	|90,000	|
|2	|32K	|500,000	|500	|network optimized	|110,000	|110,000	|
|3	|32K	|500,000	|500	|network optimized	|215,000	|215,000	|
|	|	|	|	|	|	|	|
|1	|64K	|100,000	|500	|network optimized	|30,000	|57,000	|
|2	|64K	|100,000	|500	|network optimized	|69,000	|61,000	|
|3	|64K	|100,000	|500	|network optimized	|120,000	|120,000	|
|4	|64K	|100,000	|500	|network optimized	|115,000 - 175,000	|175,000	|
|5	|64K	|100,000	|500	|network optimized	|115,000 - 165,000	|230,000	|

---------

Signed-off-by: Alexander Shabanov <[email protected]>
Signed-off-by: xbasel <[email protected]>
Signed-off-by: Madelyn Olson <[email protected]>
Co-authored-by: Alexander Shabanov <[email protected]>
Co-authored-by: Madelyn Olson <[email protected]>
Signed-off-by: chzhoo <[email protected]>

### Overview
This PR introduces the ability to avoid copying the content of string
object into replies (i.e. bulk string replies) and to allow I/O threads
refer directly to obj->ptr in writev iov.

### Key Changes
* Added capability to reply construction allowing to interleave regular
replies with copy avoid replies in client reply buffers
* Extended write-to-client handlers to support copy avoid replies
* Added copy avoidance of string bulk replies when copy avoidance
indicated by I/O threads
* Minor changes in cluster slots stats in order to support
`network-bytes-out` for copy avoid replies
* Copy avoidance is beneficial for performance despite object size only
starting certain number of threads. So it will be enabled only starting
certain number of threads. Internal configuration
``min-io-threads-copy-avoid`` introduced to manage this number of
threads

**Note**: When copy avoidance disabled content and handling of client
reply buffers remains as before this PR

### Implementation Details
####  ``client`` and  ``clientReplyBlock`` structs:
1. ``buf_encoded`` flag has been added to ``clientReplyBlock`` struct
and to ``client`` struct for static ``c->buf`` to indicate if reply
buffer is in copy avoidance mode (i.e. include headers and payloads) or
not (i.e. plain replies only).
2. ``io_last_written_buf``, ``io_last_written_bufpos``,
``io_last_written_data_len`` fields added ``client`` struct to to keep
track of write state between ``writevToClient`` invocations
####  Reply construction:
1. Original ```_addReplyToBuffer``` and ```_addReplyProtoToList``` have
been renamed to ```_addReplyPayloadToBuffer``` and
```_addReplyPayloadToList``` and extended to support different types of
payloads - regular replies and copy avoid replies.
3. New ```_addReplyToBuffer``` and ```_addReplyProtoToList``` calls now
```_addReplyPayloadToBuffer``` and ```_addReplyPayloadToList``` and used
for adding **regular** replies to client reply buffers.
4. Newly introduced ```_addBulkOffloadToBuffer``` and
```_addBulkOffloadToList``` are used for adding **copy avoid** replies
to client reply buffers.

####  Write-to-client infrastructure:
The ```writevToClient``` and ```_postWriteToClient``` has been
significantly changed to support copy avoidance capability.

####  Debug configuration:
1. ``min-io-threads-avoid-copy-reply`` - Minimum number of IO threads
for copy avoidance
2. ``min-string-size-avoid-copy-reply`` - Minimum bulk string size for
copy avoidance when IO threads disabled
3. ``min-string-size-avoid-copy-reply-threaded`` - Minimum bulk string
size for copy avoidance when IO threads enabled

### Testing
1. Existing unit and integration tests passed. Copy avoidance enabled on
tests with ``--io-threads`` flag
2. Added unit tests for copy avoidance functionality

### Performance Tests

Note: pay attention `io-threads 1` config means only main thread with no
additional io-threads, `io-threads 2` means main thread plus 1 I/O
thread, `io-threads 9` means main thread plus 8 I/O threads.

#### 512 byte object size
Tests are conducted on memory optimized instances using:
* 3,000,000 keys
* 512 bytes object size
* 1000 clients

|io-threads (including main thread)	|Plain Reply	|Copy Avoidance	|
|---	|---	|---	|
|7	|1,160,000	|1,160,000	|
|8	|1,150,000	|1,280,000	|
|9	|1,150,000	|1,330,000	|
|10	|N/A	|1,380,000	|
|11	|N/A	|1,420,000	|

#### Various object size, small number of threads

|iothreads |Data size |Keys |Clients |Instance type |Unstable branch
|Copy Avoidance On |
|---	|---	|---	|---	|---	|---	|---	|
|1	|512 byte	|3,000,000	|1,000	|memory optimized	|195,000	|195,000	|
|2	|512 byte	|3,000,000	|1,000	|memory optimized	|245,000	|245,000	|
|3	|512 byte	|3,000,000	|1,000	|memory optimized	|455,000	|459,000	|
|4	|512 byte	|3,000,000	|1,000	|memory optimized	|685,000	|685,000	|
|	|	|	|	|	|	|	|
|1	|1K	|3,000,000	|1,000	|memory optimized	|185,000	|185,000	|
|2	|1K	|3,000,000	|1,000	|memory optimized	|235,000	|235,000	|
|3	|1K	|3,000,000	|1,000	|memory optimized	|450,000	|450,000	|
|	|	|	|	|	|	|	|
|1	|4K	|1,000,000	|1,000	|network optimized	|182,000	|187,000	|
|2	|4K	|1,000,000	|1,000	|network optimized	|240,000	|238,000	|
|	|	|	|	|	|	|	|
|1	|16K	|1,000,000	|500	|network optimized	|100,000	|120,000	|
|2	|16K	|1,000,000	|500	|network optimized	|140,000	|140,000	|
|3	|16K	|1,000,000	|500	|network optimized	|275,000	|260,000	|
|	|	|	|	|	|	|	|
|1	|32K	|500,000	|500	|network optimized	|57,000	|90,000	|
|2	|32K	|500,000	|500	|network optimized	|110,000	|110,000	|
|3	|32K	|500,000	|500	|network optimized	|215,000	|215,000	|
|	|	|	|	|	|	|	|
|1	|64K	|100,000	|500	|network optimized	|30,000	|57,000	|
|2	|64K	|100,000	|500	|network optimized	|69,000	|61,000	|
|3	|64K	|100,000	|500	|network optimized	|120,000	|120,000	|
|4	|64K	|100,000	|500	|network optimized	|115,000 - 175,000	|175,000	|
|5	|64K	|100,000	|500	|network optimized	|115,000 - 165,000	|230,000	|

---------

Signed-off-by: Alexander Shabanov <[email protected]>
Signed-off-by: xbasel <[email protected]>
Signed-off-by: Madelyn Olson <[email protected]>
Co-authored-by: Alexander Shabanov <[email protected]>
Co-authored-by: Madelyn Olson <[email protected]>
Signed-off-by: shanwan1 <[email protected]>

…addRepliesWithOffloadsToList (#2201)

Replaced direct cast to robj** with memcpy to avoid undefined behavior
on unaligned buffers, which caused test failure when built with UBSan.

Follow-up of #2078

Signed-off-by: xbasel <[email protected]>

Because valkey optimizes the reuse of client buffers, so the test will
break.
see valkey-io/valkey#2078

…addRepliesWithOffloadsToList (valkey-io#2201)

Replaced direct cast to robj** with memcpy to avoid undefined behavior
on unaligned buffers, which caused test failure when built with UBSan.

Follow-up of valkey-io#2078

Signed-off-by: xbasel <[email protected]>

In valkey-io#2078, we did not report large reply when copy avoidance is allowed.
This results in replies larger than 16384 not being recorded in the
commandlog large-reply. This 16384 is controlled by the hidden config
min-string-size-avoid-copy-reply.

Signed-off-by: Binbin <[email protected]>

In #2078, we did not report large reply when copy avoidance is allowed.
This results in replies larger than 16384 not being recorded in the
commandlog large-reply. This 16384 is controlled by the hidden config
min-string-size-avoid-copy-reply.

Signed-off-by: Binbin <[email protected]>

…#2652)

In valkey-io#2078, we did not report large reply when copy avoidance is allowed.
This results in replies larger than 16384 not being recorded in the
commandlog large-reply. This 16384 is controlled by the hidden config
min-string-size-avoid-copy-reply.

Signed-off-by: Binbin <[email protected]>

In #2078, we did not report large reply when copy avoidance is allowed.
This results in replies larger than 16384 not being recorded in the
commandlog large-reply. This 16384 is controlled by the hidden config
min-string-size-avoid-copy-reply.

Signed-off-by: Binbin <[email protected]>