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

This is the only case where the "maybe" applies, right? In the other places, the checks that are in ```MaybeUpdateLocalServingSTH``` are done outside, I was finding this a little weird that it sounded like the change might still get ignored... Maybe the, er, ```Maybe``` could be dropped, the ```if``` inside be converted to a ```CHECK```, and an ```if``` added here?

I'm also not sure how I feel about **not** following what ```etcd``` says the serving STH should be? I guess the situation where that could happen is if there's two calls to ```SetServingSTH``` in a row, which would have set the local serving STH to the one with the highest timestamp, and now the updates would be flowing back in... Maybe ```SetServingSTH``` should **not** set the local one, and rather let it be set from the watch updates?