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

You're not using this for anything else than the two tasks, so there wouldn't be any need to keep it...

I know I've been putting ```shared_ptr``` in a lot of things, but I'm not sure that was a great idea... Doing it the "classic" way, just taking a pointer, and saying that it has to outlive this object, might be a bit simpler? My worry is for things that can introduce self-references, especially as ```std::bind``` does the "right thing" with them (a bound callback that involves a ```shared_ptr``` takes a copy, so it counts as a reference).

If you were doing more things with ```base_```, I could be convinced, but since it's just for the tasks, I'd say you could just take a plain ```util::Executor* executor```, and pass it down to the tasks without keeping it.