Initial fixes for leaking Netty connections on ActorSystem shutdown

Fixes #148

These are the initial workarounds that I tested locally for fixing the Netty connections that were leaking when the ActorSystem was shutdown without sending Close to the MongoConnection. I simply copied the "Close" behavior for MonitorActor and MongoDBSystem into the relevant postStop() methods.

Some de-duplication work and some extra unit tests would of course be a good idea but I haven't had time for that yet.