Currently, if heartbeat fails, we reschedule it after 5 seconds.
This is fine for the first retry, but it can cause a thundering herd
problem when a lot of nodes fail to heartbeat at once.
This change adds jitter to the minimum wait of 5 seconds. The jitter is
not applied for forced heartbeats: they still have a minimum wait of
exactly 5 seconds from the last heartbeat.
The code is re-ordered to move the interval calculation to one place.
Bonus: correctly logging the next interval.
The unit tests have been rewritten to test the heartbeat process step by
step and not rely on the exact sequence of the calls.
Closes-Bug: #2038438
Change-Id: I4c4207b15fb3d48b55e340b7b3b54af833f92cb5
2ab8364649