R1:782b18e7e2ed

Migrate RefCursor data to RefPosition table

Summary:
Ref T11823. This populates the new RefPosition table based on the existing RefCursor table, and deletes now-duplicate rows in the RefCursor table so the next change can add a unique key.

This change is not standalone, and there need to be separate code updates. I have a rough version of that written, but this migration needs to happen first to test it.

I'll hold this whole series of changes until after the release cut and until the code is updated.