Run Repository Check (`git fsck`) parallel for each shard
At the moment
RepositoryCheck::BatchWorker runs once every hour checking all the repos one by one.
We could improve the throughput drastically, without increasing the load on the servers, if we run a
BatchWorker for each shard. So
git fsck will never run more than once on 1 storage server, but for gitlab.com it would still multiply the throughput by factor 16.