This converts the MergeRequestCleanupRefsWorker to be a limited
capacity worker.

The ScheduleMergeRequestCleanupRefsWorker will still be enqueuing
jobs but it'll be based on capacity. If there's no capacity then
no new jobs will be enqueued.

For now, we are capping it at 4 jobs at a time. This will be later
on be configurable.

This is still behind the merge_request_refs_cleanup feature flag
so we can easily turn it off on production while testing.

Changelog: changed