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
You can reduce the nesting (and eliminate the scope braces) here, by doing an early return:

if (replicaSetMonitorWatcher)
    return;

boost::lock_guard<...>