Kill StaticObserver and enable start/stop of ReplicaSetMonitorWatcher

This is slightly reworked version of the initial PR. 

- StaticObserver is no more
- added initialize() method to ReplicaSetMonitor
- call initialize() where needed in tests
Yeah that would be some double-checked-locking style badness. Anyway, we've discussed it enough. What you have is correct, so up to you if you want to change (assuming that holding the setsLock while making the RSMW is ok: I don't actually know if it is).