A schedule's claim was kept when its run was cut off, so after a
restart every instance skipped it for most of an interval. The scheduler
now drops the claim when the run ends with its context cancelled.
Instances started together on an empty database created the schema at the
same time and all but one failed on duplicate types. Schema creation and
migrations now run under a Postgres advisory lock.
Eviction, reclaim, retention and the Gradle build cache sweep take a
per-job advisory lock for each pass, and an instance that finds the job
running elsewhere skips the pass. Before, every instance evicted the whole
overshoot over max_size at once and emptied the cache.
Mirror schedules record their last start in a new job_runs table, so each
schedule runs once per interval instead of once per instance.
Refs #543
* 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.
