Batch gitaly delete refs in buffered queue

As described in gitaly#5369 (closed) we should batch our delete refs together in batches per project. Deleting a single ref in Gitaly is about as fast as deleting 100 refs so it's much more efficient for very busy projects (where refs are deleted at a very high rate by pipelines finishing, MRs merging) to batch them together. This is especially important to improve throughput now that we had to serialize deletes per project in order to avoid race conditions that led to deadlocks.