HA: detect when a node fails without warning

When a node goes AWOL - absent without leave, i.e. it has gone away
unexpectedly without notifying anyone - this change will detect if the
heartbeat period has expired and implicitly set the node state to
FAILED.
Fix + test HA Manager inferring failed nodes
Merge pull request #1456 from richardcloudsoft/ha-detect-failure

HA: detect when a node fails without warning