High number of Sidekiq jobs in use_key and git_garbage_collect workers

• use_key is used anytime the /allowed endpoint is hit via a git pull/clone (gitlab-org/gitlab-ce!8113)
• We thought git_garbage_collect would be better after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12529, but do we need to check HousekeepingService#service_needed? From Slack:

Yorick: @#$(*&#$(* somehow I overlooked that when reviewing the MR
To be fair, I think that `needed?` check really should be embedded in `execute`
otherwise we will keep making the same mistake

What can we do better here?

/cc: @smcgivern, @dbalexandre