StopNotify

Add `StopNotify` to `etcdsever`. 
So the launcher of `etcdserver` can be notified when `etcdserver` is stopped. 
This has nothing to do with `stopNotify`, I am mocking the `stop` logic here.
Do not worry about this right now. I plan to move all `stop` related stuff to a separate pkg called `stop` or something like that. or at least we figure out a way to test the whole `stop` logic.