Add background job to clear unreferenced state groups

[devonh]

Fixes #18150

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
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• Code style is correct
  (run the linters)

[devonh]

I still need to add tests, but otherwise this is currently working.

[erikjohnston]

Just a couple of minor tweaks, otherwise I think the approach is good

[devonh]

Should be good to go now!

[erikjohnston]

Woop, thanks! Sorry for it going round in circles a bit