Improve Shutdown Reliability

* Make MongoDriver always use its own ActorSystem for increased isolation from other ActorSystems
* MongoDriver now will not initialize from a 3rd party actor system
* Add a Supervisor actor to MongoDriver to supervise shutdown of the driver and its ActorSystem
* Give names to Monitor and Connection actors for easier tracking
* Make Monitor/Connection/Supervisor actor names unique for better debugging
* Make MongoDriver.close exit cleanly by waiting for ActorSystem to shutdown
* When MongoDBSystem is closing, if it gets a ChannelConnected, just close the channel and log a warning
* Add a test case to test the above.
* Various documentation fixes to quiet down warnings a bit.