R1:3ad727ba787c

Guarantee the `key_position` key is created properly

Summary:
Ref T12987. I was focused on the RefCursor table and overlooked that we need some care on this key.

It's currently possible to run `bin/storage upgrade --no-adjust`, then start Phabricator, and end up with duplicate records in this table. If you try to run `bin/storage adjust` later, it will try to add the unique key but fail. This is unusual for normal installs (they usually do not use `--no-adjust`) but we do it in the cluster and I did this…