Make backoff delay configurable
### Problem to solve
We are using GitLab mainly to manage our git repositories (no ci, no issues, ...). Historically if someone pushes to a repo, these changes should be available at some other host. We are using the *Mirroring repositories* feature, to push these changes (Community Edition).
If someone pushes "to fast" the backoff delay delays the push to the mirror (1 minute).
### Proposal
The backoff delay should be configurable in seconds (one minute is still too long), ideally via the GitLab webinterface.
### Links / references
https://gitlab.com/gitlab-org/gitlab-ee/issues/3832
https://gitlab.com/gitlab-org/gitlab-ee/commit/8eba4eb6262b1bd5565fd832eb56e01af006c885
issue