Forces import worker with mirror to insert mirror in front of queue
In response to the following discussion here: https://gitlab.com/gitlab-com/infrastructure/issues/2083#note_33113583
Also changed the lease cancelation in UpdateAllMirrorsWorker
to be cancelled when the maximum capacity was reached. This is to make it so that the next scheduler will be able to check again instead of waiting 5 minutes for the lease to expire.