chore: prepare v0.2.21 release (tokio-rs#2530)

chore: prepare v0.2.20 release. (tokio-rs#2458)

chore: prepare v0.2.19 release (tokio-rs#2441)

chore: prepare to release 0.2.18 (tokio-rs#2399)

chore: prepare to release 0.2.17 (tokio-rs#2392)

# 0.2.17 (April 9, 2020)

### Fixes
- rt: bug in work-stealing queue (tokio-rs#2387) 

### Changes 
- rt: threadpool uses logical CPU count instead of physical by default
  (tokio-rs#2391)


Signed-off-by: Eliza Weisman <[email protected]>

- sync: fix a regression where `Mutex`, `Semaphore`, and `RwLock` fut…

…ures no

  longer implement `Sync` (tokio-rs#2375)
- fs: fix `fs::copy` not copying file permissions (tokio-rs#2354)

- time: added `deadline` method to `delay_queue::Expired` (tokio-rs#2300)
- io: added `StreamReader` (tokio-rs#2052)

chore: Prepare `0.2.15` release (tokio-rs#2365)

Signed-off-by: Lucio Franco <[email protected]>

chore: prepare tokio v0.2.14 release (tokio-rs#2356)

chore: prepare v0.2.13 release (tokio-rs#2282)

Includes a quick bug fix

chore: prepare v0.2.12 release (tokio-rs#2278)

Also includes `tokio-macros` v0.2.5.