git/housekeeping: Prune stale multi-pack-index locks

We have recently heard of a customer for whom repository housekeeping stopped working: even though OptimizeRepository() is running, it always returns an error when repacking objects.

As it turns out, the root cause of this is that there was a stale multi-pack-index lock that makes Git fail. Amend our housekeeping logic to prune such stale locks so that Gitaly can eventually recover without the help of an administrator.

Closes #5344 (closed).