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).
If I get old STH, update_sth failed and I know it, so I can simply return false. I run verify_consistency to make sure that log is still consistent (because it's ok for log server to return older sth, but it's never ok to return inconsistent one).