Sidekiq infra for pull mirrors

RE: https://gitlab.com/gitlab-org/gitlab-ee/issues/3021#note_36460602

@pcarranza Can I please get Sidekiq infra for pull mirrors? :)

For this first iteration, ideally we'd get concurrency of 20: 20 mirrors being updated at the same time.

You likely have a better idea of the number of hosts, and CPU/RAM required.