* Version bump for 4.8.1 Release

Updated the version numbers of pods which are being released in 4.8.1

* Update from master

Picking up a few last-minute changes to CHANGELOGs and tests for 4.8.1
release.

* Update Core info for M21.1

Update missing version bump for M21.1 for FirebaseCore.

* Remove over-aggressive assert.

closeWithFinalState: assumes delegate != nil, but that is not true if when
startWithdelegate: was called we entered backoff (performBackoffWithDelegate:)
and so self.delegate did not get assigned yet.

We could rework the code to make the assertion hold, but per offline
discussion this assert doesn't represent an invariant that we care about
and so I'm just removing it.

* Remove over-aggressive closeWithFinalState: delegate assert. (#656)

Fixes #596. closeWithFinalState: asserted delegate != nil, but that is not true if when
startWithdelegate: was called we entered backoff (performBackoffWithDelegate:)
and so self.delegate did not get assigned yet.

We could rework the code to make the assertion hold, but per offline
discussion this assert doesn't represent an invariant that we care about
maintaining and so I'm just removing it.

* Increase FirebaseAuth version for M21.1

This version was missed in the M21.1 version bump PR.

* Fix import formatting (#660)

* Fix issue @morganchen12 discovered where we weren't properly creating FIRQueryDocumentSnapshot instances. (#662)

* Validate clang-format compliance in travis (#648)

* Build gRPC for Firestore C++ (#652)

* Clean up quoting and other minor issues

* Reorganize CMake build output

Make it clearer which parts of the output pertain to external projects.

* Use a consistent ordering of ExternalProject arguments

* Prevent the top-level build from running in parallel

This prevents spurious failures when running make -j.

* Actually parse arguments in the xcodebuild function

* Use ExternalProject features when available

  * submodule limits from CMake 3.0
  * shallow clones from CMake 3.6
  * git progress output from CMake 3.8

* Only build the parts of leveldb we need

Skip building the tools and other libraries

* Avoid installing ExternalProjects

Consume build output directly so that we can build just the targets we
need. Installing causes all targets to be built.

This doesn't matter as much for these targets but the gRPC build
includes a ton of stuff we don't need so it's worth adopting this as a
general strategy.

* Define an external build for grpc

* Test that grpc can link successfully.

* Add a FindGRPC CMake module

* Actually comment ExternalProjext_GitSource

* Inject infoDictionary to fix flakey tests. (#664)

* Inject infoDictionary to fix flakey tests.

* Remove outdated comment, update format.

* Fix issue @morganchen12 discovered where we weren't properly creating FIRQueryDocumentSnapshot instances. (#662)

* Travis - run tests only for changed code (#665)

* Add assert_test to the Xcode build (#671)

* Exclude stdio-backed assert from the Xcode build

* Add assert_test to the Xcode build

* Enable warnings in the CMake build (#669)

* Enable warnings when building with GCC or clang

* Fix warnings

* Fix C++ lint errors (#668)

* Misc style.sh fixes

  * Allow test-only to use a revision; to check your changes since
    master:

    ./scripts/style.sh test-only master
  * Avoid diffing deleted files
  * 80 columns

* Fix C++ lint errors

* implement FieldValue for null, boolean, and array in C++. (#637)

* implement FieldValue for null and boolean.

* refactoring to use union type instead of poly

* refactor using union design intead of poly

* refactoring to use anonymous union and fix styles

* small fix

* add field_value_test to the project

* fix warning of cmake and fix style

* Simplify integration with googletest (#672)

This makes it possible to build the Firestore subproject with CLion
because it no longer needs to be told where googletest is.

* Add a cc_library to the CMake build (#670)

* Rewrite cc_test to take named arguments

Cut down on build file verbosity by having cc_test take SOURCES and
DEPENDS. The separate invocation of target_link_libraries is no longer
necessary.

* Add a cc_library rule to parallel cc_test

This cuts down on build file verbosity.

* Automatically add OBJC_FLAGS to cc_libraries if applicable

* Exclude platform-specific libraries from 'all'

This is makes it possible to declare this kind of library
unconditionally. Usage within a test or as a dependency will actually
trigger building.

* Restore secure_random_test.cc; clean-up comments

* Fix CMake build and lint warnings in field_value.cc (#677)

Fixes these cpplint warnings:

Firestore/core/src/firebase/firestore/model/field_value.cc:162:  Semicolon defining empty statement. Use {} instead.  [whitespace/semicolon] [5]
Firestore/core/src/firebase/firestore/model/field_value.cc:170:  Semicolon defining empty statement. Use {} instead.  [whitespace/semicolon] [5]
Firestore/core/src/firebase/firestore/model/field_value.cc:126:  Add #include <utility> for swap  [build/include_what_you_use] [4]

* Listen sequence numbers (#675)

* Generate and save sequence numbers for listens

* Add documentation

* Fix include path

* Fix unavailable comment

* Review feedback

* Fix build warnings by making void parameter explicit (#681)

* Add platform detection logic for SecureRandom (#676)

* Add CMake platform detection logic for SecureRandom

Now only builds secure_random_arc4random.cc if available.

Remove firebase/firestore/base/port.h. Nothing else was in that
directory.

* Add a SecureRandom implementation that uses OpenSSL

This is usable on Linux, Windows, and Android

* Properly check return from RAND_bytes

* Port comparison to C++ (#678)

This reimplements our comparison functions as C++ Comparators and then provides compatibility shims for interoperating with existing Objective-C usage.

A few specialized comparators aren't suitable for porting but only have a single usage (e.g. CompareBytes for comparing NSData * instances). In these cases I've moved them into the caller.

* Use int32_t for typeof(ID) in FSTDocumentReference

* Migrate callers of FSTComparison.h to Objective-C++

* Port comparison to C++

* Migrate usages of FSTComparison.h to C++ equivalents

* Remove FSTComparison

* Use Comparator in FieldValue. (#686)

* style.sh - output unformatted files in test-only and ignore generated config.h (#690)

* Cleanup imports and isEqual (#685)

* Fix headers

* Fix isEqual verbosity

* Fix isEqual for nullable properties

* Fix nullability on FSTTestDocSnapshot

* Fixing spelling error in FIRStorageErrors

* Update travis to use CocoaPods 1.4.0 (#692)

* add FIRUser behavior to documentation

* Disable Messaging build warnings (#697)

* Update FirebaseMessaging protos (#696)

* Adding enable/disable property to FCM token auto Initialization

* revert unrelated changes

* revert unrelated changes

* revert unrelated changes

* Adds explicit core graphics dependency to auth (#700)

* Properly publish Abseil sources as a part of the podspec (#704)

* Properly include abseil sources
* Exclude abseil tests

* Implement the rest of FieldValue types for C++ (