Fix data race when reading conn.isClosed
Merge pull request #142 from Zariel/fix-isclosed-data-race

Fix data race when reading conn.isClosed
Added check to prevent user from executing a use statement from an active session.
Updated README.md with examples of defining the default keyspace correctly for a session.
Cleaned up code to streamline the connection being added to the pool and removed code related to old keyspace behaviour.
Merge pull request #144 from phillipCouto/prevent_use_keyspace

Prevent use keyspace
(cleanup) breaking out compressor from conn
Merge pull request #147 from harpreet-sawhney/master

(cleanup) breaking out compressor from conn