Serialize delete refs for Merge Requests to avoid Gitaly deadlock

What does this MR do and why?

Serialize delete refs for Merge Requests to avoid Gitaly deadlock.

This is a fix similar to !123976 (merged). It changes both to use the same serialization mechanism to reduce the likelity of Gitaly deadlock as described in gitaly#5368 (closed).

This MR does:

• Build on top of work in !123976 (merged).
• Serializes all references managed by merge-requests: refs/merge-requests/IID/head, refs/merge-requests/IID/merge, refs/merge-requests/IID/train.
• They are scheduled and run async, synchronized with deleted being done by CI Pipelines.
• This should cover 99.99% of cases when deletes does happen in code.

Remarks:

• Those deletes still happen one-by-one, so they do generate extra pressure on Gitaly.
• This does show case all places where deletes do happen and generalises their handling making it easier to implement: gitaly#5369 (closed).
• This has its own feature flag: merge_request_cleanup_ref_worker_async. To avoid deadlock the merge_request_cleanup_ref_worker_async and pipeline_cleanup_ref_worker_async should be enabled.

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(: merge_request_cleanup_ref_worker_async)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.