Add the ability to watch the serving STH and the cluster node state objects to EtcdConsistentStore.

I was thinking about this, and this is potentially racy (two threads could check, both see that it wasn't notify, and both try to notify it, the loser causing the program to CHECK-fail). I think that at the moment, ```EtcdClient``` will only issue a single watch callback at a given time, but I'm not sure if that's true with the fake one, and we didn't really document that in the interface, so it could conceivably change in the future...