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.
Maybe it'd be useful to have a log message here saying that it was successfully stopped