Move background tasks out of the main process

[clokep]

Create a new worker type which runs background tasks, many of which are periodic. See #7670 for some rationale.

[clokep]

Related to #7593.

[clokep]

Things left to do:

• Move this to the background worker.
• Check if _maybe_retry_device_resync can be run on any worker.
• Enable this on matrix.org.

[clokep]

Check if _maybe_retry_device_resync can be run on any worker.

The conclusion on this was that it needs to run on whatever is the device list writer, which is currently the main process.

[clokep]

This is deployed and seems to be working. I think this is done! 🎉