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
