`RepositoryUpdateMirrorWorker` are overeager to schedule new `UpdateAllMirrorsWorker`s

If there is a lot of capacity, RepositoryUpdateMirrorWorkers will schedule new UpdateAllMirrorsWorkers every 2 seconds until we catch up with capacity, which is excessive, because catching up may take a while (or may never happen). They could also check UpdateAllMirrorsWorker's lease to know if a new one is needed. Or it may make more sense for UpdateAllMirrorsWorker to schedule another one of itself if it detects leftover capacity.

This is not necessarily a problem because we already use a lease to ensure only one UpdateAllMirrorsWorker is running at a time, but we can tone it down a little.

/cc @tiagonbotelho