[1094540] Agents get permanently backfilled if backend database goes offline for a moment
Clean up some of the live availability doco/workflow:
- Fix the jdoc for DiscoveryAgentService.getCurrentAvailability() to reflect reality
- Fix ResourceManagerBean.getLiveResourceAvailability() to mark its avail report
  as a "ServerSideReport".  This prevents it from interfering with server-agent
  backfill coordination.  Also, correct some confusing inline doco and doe some cleanup.
- Fix AvailabilityManagerBean.mergeAvailabilityReport() such that logic checking for the
  backfill flag does not execute *after* the backfill flag has been reset. And clean up
  the overall logic a bit more.
- Improve jdoc for AvailabilityManagerLocal.updateLastAvailabilityReportInNewTransaction
  to clearly indicate its side-effect of clearing the backfill flag.