Runs the jobs of one queue, up to concurrency of them at a time.

A queue is started for every entry in the :queues configuration and checks the database every check_interval milliseconds for work to pick up, in this order: workers left behind by a previous run, then scheduled, retrying and enqueued workers that are due, oldest first.

Each job runs in its own process, supervised by - and monitored by - the queue, which is how a slot is released: when a worker process terminates, for any reason, the queue frees its slot and looks for more work right away. A worker that terminates without recording its own result is retried or failed by the queue.