Check disconnections on connection reacquiry (#145)

Check disconnections on connection reacquiry (#145)

* Detect EOF signaling remote server closed connection

Raise ConnectionClosedByRemote and handle on `send`

* Fix linting

* Use existing NotConnected exception

* Add `Reader.is_connection_dropped` method

* Check connection before sending h11 events as well

* Add test covering connection lost before reading response content

* Check for connection closed on acquiring it from the pool

* Clean up ConnectionPool logic around reaquiry of connections