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).
As discussed offline, it will be simpler to return None in case something went wrong with get_entries and the number of entries (could be 0) if get_entries succeeded.