We saw in production that DispatchWorker was running about twice an hour,
which would schedule twice as many jobs as it should.

For some reason, BatchWorker was running 1000 times per hour, possibly
due to Sidekiq RSS kills that caused these jobs to restart.

Adding an ExclusiveLease prevents these jobs from running more
than they should.

Relates to https://gitlab.com/gitlab-com/infrastructure/issues/4526

The RepositoryCheck::DispatchWorker will start a
RepositoryCheck::BatchWorker for each healthy shard.

Closes gitlab-org/gitlab-ce#48042