ADM-62937 Make heartbeater resilient to clock tampering.
If the system clock changes by more than the heartbeat interval (default 1000ms)
between heartbeats, then compensate for this one-off difference before removing
members from the cluster.  Also log a warning if the clock jump is > 5000ms.
ADM-62937 Improve time formatting
ADM-62937 Get seconds -> milliseconds conversion right
Merge pull request #25 from atlassian/ADM-62937-make-heartbeater-resilient-to-clock-tampering

ADM-62937 Make heartbeater resilient to clock tampering.