ConnectionPool checkin should put connection back into the pool

(this PR depends on #226 being merged first)

We need to checkin the connection(put back in the pool) after use.
We cannot only depend on the reaper to get rid of connection, as threads could be sleeping or blocked by something else. If we make the connections rotate in the pool, we could have more threads using less connections.

just for an example, other gems are going the same thing:
https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb#L370
https://github.com/zk-ruby/zk/blob/master/lib/zk/pool.rb#L203
https://github.com/mperham/connection_pool/blob/master/lib/connection_pool.rb#L79

[fixes #223]

review @durran