* fix `Pool` to reduce possibility of "leaking" connections
now uses RAII guards to control `SharedPool::size`
* add smoke test for `Pool` to both Postgres and MySQL tests
add `Pool::is_closed()`
* fix documentation re: pool
* refactor pool implementation to not use futures oneshot channels
https://github.com/launchbadge/sqlx/pull/84#issuecomment-580476223
* run cargo fmt
* Pool: remove superfluous guard struct, document some internal methods