[Reviewer: Andy] Check that the current time is more recent than the service time

Andy, can you review this change to check that the current time is more recent than the last serviced time

Will fix [Sprout #712] (https://github.com/Metaswitch/sprout/issues/712)
This can still false-positive if the latency is `_deadlock_threshold - 1` second, worth a comment/1 second's grace period?