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, the return value of this lambda should be a deferred (that may have already fired if we have pending sth) so it will be waited on before _update_result is called.