Order delivery of activities by last seen

@evanp made a good point at FediCon this weekend that staggered and delayed delivery of activities over AP is fine because not all recipients need to get the activity right away.

A simple way to ensure that more relevant users receive the activity first, is to order the activity by last seen time by the instance. If someone hasn't been seen for weeks, real-time (or even just timely) delivery of an activity to that server is much less important.