Move reachability notification triggering out of isReachableChanged into
a dedicated ServerReachabilityChanged event dispatched by
ServerConnectionCheckJob. Remove the blocking 3-attempt sleep loop from
isReachableChanged — unreachable_count threshold alone now gates the
Unreachable notification. Add feature and unit tests covering all
notification dispatch paths.
Reduce load on unreachable servers by implementing exponential backoff
during connectivity failures. Check frequency decreases based on
consecutive failure count:
0-2: every cycle
3-5: ~15 min intervals
6-11: ~30 min intervals
12+: ~60 min intervals
Uses server ID hash to distribute checks across cycles and prevent
thundering herd.
ServerCheckJob and ServerConnectionCheckJob increment unreachable_count
on failures. ServerManagerJob applies backoff logic before dispatching
checks. Includes comprehensive test coverage.
