Previously, UpdateAllMirrorsWorker would only reschedule itself when
enough mirrors were waiting to sync to fill up all available capacity.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Customers using remote mirrors would see errors in their Sidekiq logs after
the worker attempted to update its database.  We should disable this until we
have a better way of synching the mirror workers.

See #1598

If you attempt to run this spec between 00:00 and 01:00, one of
the examples fails because:

DateTime.now.beginning_of_hour == DateTime.now.at_beginning_of_day

Closes #2081
Signed-off-by: Rémy Coutable <remy@rymai.me>