Revert "Updating derived collections are now not scheduled"

This reverts commit ba5067b8c4d45ff6ad28c0a85c6d5c31241f3836.
ItemChanged must be scheduled or else it gets really hard to reason about
I feel shame as a minor action
Merge pull request #523 from reactiveui/rxcoll-must-be-scheduled

ReactiveList.ItemChanged must be scheduled