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).
Nit: Add to the logging statement the amount of time to sleep, so when looking at the monitor logs it's clear if it's stuck or just pending another probing round.