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

As an example of the "maybe" being a bit strange... If we just updated the serving STH above, throwing out the handle (which is the only thing we really need, for the next update to work correctly) away would always be wrong, no? If we were going to throw it away, might as well ```CHECK```, since we won't be able to update it again...