follow metrics change in EL freshness alert

Now that tpo/network-health/metrics/monitoring-and-alerting!19 (merged) has been merged, the old metric will stop being updated and will actually fail to alert (and silently too!).

Instead, we check the number of times the update timestamp changed in the last predefined period (3 hours or 180 minutes). If that's lower than zero, that means the lists are stale, so we alert.

Closes: #12 (closed)