Log more information about statestore failure detector

For debugging heartbeat failures (or non-failures) it would be useful to log enough information to infer the current state of the failure detector from logs. Specifically:

• Upon a failure, we should log the number of consecutive failures according to the failure detector. And also maybe how many failures remain until it's considered to be failed.
• We should log when the failure count is reset to 0 by a successful heartbeat.

Currently if there are occasional failures it's hard to tell with certainty whether it was reset correctly.