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
Ah, well, now that you removed the other part, I like it better the other way! You can just make it say

```
boost::lock_guard<boost::mutex> lock(watcherLifetimeLock);
if (replicaSetMonitorWatcher) {
    replicaSetMonitorWatcher.reset(...);
```