When a new device list change occurs, we're now:
1. For each appservice, checking the last device list stream key that was
processed up until.
2. Getting any users with changed device list between the last device list
stream key and the stream key of the triggering update.
3. Filtering out those users based on those that are actually relevant
to this application service.
4. Passing those changes to enqueue_for_appservice and saving the device list
stream key that we've just processed up to for later reference.
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