This does two things:

• Respect the rather new Defaults.run_async when deciding if update_persistence should be called or not. This was just overlooked by me in Add Defaults.run_async support #2210
• call update_persistence only once per update and only if the update was handled by at least once sync handler. Having a update handled by multiple groups can slow down the bot significantly otherwise.

Also this actually documents how stuff is updated (and fixes the annotation of process_update on the fly)