Async monitor

Changed Monitor, so it works with AsyncLogClient. Also added get_sth_consistency to async client. This is first step in making update_entries faster (by making few asynchronous fetches). I also changed all other affected files accordingly (unit tests and prober).
Describe why you need this function, not what it's done (which is fairly obvious from the code):
"Create a fired deferred to indicate that the asynchronous operation should proceed immediately, when the result is already available."