This is more for the (unusual) case where you are running a pan demon for a larger chunk of users, and want to conserve resources by not running a sync perpetually for all your users at once. Have a reasonable timeout where if there is no activity from the client after a given number of seconds, we should treat them as offline and not sync.

I would expect that the default would be to sync perpetually, but it would be nice to have this option. Relatedly, it would be good to not sync all users on startup but rather on demand.