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
I suspect that this method should both:
- Return a Status
- Take a shutdown grace period
- Use that grace period in replicaSetMonitorWatcher->wait();

Only if the wait is OK with the timeout should you go on to cleaning up state.