Delete rows from `device_inbox` in the background when deleting a device

[erikjohnston]

device_inbox can have a huge number of rows when deleting a user, which makes user deletion slow and costly. Instead, we should do the deletion of rows in device_inbox as a background task (potentially using #15891).

[erikjohnston]

I think we can use the new TaskScheduler class to add a task in the background to delete all to-device messages for the device after deletion. The best way of doing that would likely be to just repeatedly do DELETE FROM device_inbox WHERE user_id = ? AND device_id = ? AND stream_ordering < ? LIMIT 100. Care should be taken to only delete to-device messages from before the deletion, to handle the case where the device logs back in again with the same device ID.