Slower sync when backoff time is being respected

We upgraded GPRD nodes to 10.6 RC6, which I thought had https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4949. But upon further investigation, it did not.

I noticed our graphs for sync time looked a lot faster:

I suspect this is because so many failed repositories have a backoff time in the future, so we're not able to schedule as many. But I would like more data to confirm.

For now, I applied https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4949 manually.