resolves#109095resolves#106854
Changes the way task manager and alerting perform their health / status
checks:
- no longer sets an `unavailable` status; now uses `degraded` instead
- change task manager "hot stats freshness" calculation to allow for
staler data before signalling a problem
- Changed the "Detected potential performance issue" message to sound
less scary, include a doc link to task manager health monitoring, and
log a debug instead of warning level
- add additional debug logging when task manager sets a status that's
not `available`, indicating why it's setting that status (in the code,
it's when task manager uses HealthStatus.Warning or Error)