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
