R1:8982e3e52dd8

Update major RefCursor callsites to work properly with RefPosition

Summary:
Ref T11823. This is the meaty part of the change, and updates `RefEngine` to use separate RefCursor (for names) and RefPosition (for actual commit positions) tables.

I'll hold this whole series until after the release cut so it has some time to bake on `secure` to look for issues. It's also not a huge problem if there are bugs here since these tables are just caches anyway, although they do feed into some other things, and…