Run background migrations with a minimum interval

What does this MR do?

This adds a minimum interval to BackgroundMigrationWorker, ensuring background migrations of the same class only run once every 5 minutes. This prevents a thundering herd problem where scheduled migrations all run at once due to their delays having been expired (e.g. as the result of a queue being paused for a long time).

If a job was recently executed it's rescheduled with a delay that equals the remaining time of the job's lease. This means that if the lease expires in two minutes we only need to wait two minutes, instead of five.

EE MR for these changes: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3933

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/41624