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.

Test Plan: Ran migration, spot-checked database tables. Saw redundant rows remove and correct-looking rows populated into the new RefPosition table.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11823

Differential Revision: https://secure.phabricator.com/D18612