* Add mirror.schedules, a list of image tags the server re-mirrors at a fixed interval. Digests are rejected since they never move.
* Run each schedule through the normal mirror, one run after the other. The first run starts at a random point within ten minutes and later runs get ±10 % jitter, so several instances don't hit the registry at once.
* Start the schedules with the server and let shutdown wait for a running refresh before storage and database are closed. Docs included.
* Check cached layers during a mirror without opening them, so a refresh no longer counts as a cache hit and doesn't keep scheduled images at the top of LRU eviction.
