Introduce SnapshotWritable for sorting by time/priority and remove PriorityTag

Part 3...

This, or something like it, makes it possible to do snapshot windowing. I hope this is the right idea, although I'm assuming there was probably a very good reason we didn't do the sorting previously.
It only works because of the grouping on SnapshotWritable, which implies the import is sorted, but it isn't sorted in this test, so it just looks a bit confusing.