Don't use bottleneck for saving to the server

Summary

This patchset changes the save manager to use a different queue implementation, because of my suspicion that some of dataloss complaints stem from bottleneck (see what I did there). I was going to rip it out entirely but then I found a much simpler (and maintained) task queue implementation so I went for that instead as I didn't fancy re-introducing the race condition.

Checklist

• Documentation impact considered
• Manual smoke test run (if possible)
• Local-only tests run no new failing tests
• Assigned related ticket to reviewer no related ticket