If a collection is dropped then immediately re-created on the primary while another node is doing an initial sync of that collection, the initial sync will fail with a CursorNotFoundError

This is because when a collection is dropped, we delete the cursors completely, so we don't know why the cursor was killed.

Possible fixes:

Include uuid of collection in cursor objects; check existence of uuid (instead of or in addition to namespace) to return a CollectionDropped error.

Keep killed cursors around for some length of time (or until the cursor is used), marked to say they were killed because the collection was dropped.