Split the start/stop actions from the river start/close methods

The RiversService will invoke #start()/#close() when the river itself is started/stopped. However
we also have our internal status (controllable via REST, and driven by the StatusChecker), which is
semi-independent from that.

Split those two lifecycle actions:
River #start()/#close() now only manages the status thread itself
MongoDBRiver #internalStartRiver()/#internalStopRiver() is invoked by the status thread

This fixes first of all #406, as we're no longer deadlocking inside the initial listener thread. As a side-effect
it also fixes a potential inconsistency between the status of the river as seen by us, and the status of the river
as seen by the RiversService.
this will block the StatusChecker and make it so that we can't stop the river, right? maybe we need to move it out into it's own thread as you were going to do before in one of your earlier PRs