Mongosync and its embedded verifier both have to very carefully handle events related to cross-shard moves. That's because the delete + insert for an update that causes a cross-shard move is split into a delete and insert event, and those two events may appear in any order in the change stream.

While it would simplify our code to ensure that the order is always delete + insert, it would be even better for us to simply see a single update event.