(PDB-898) Add factset hash to replace-facts!

This commit associates a hash of each factset with the metadata of the factset
to allow us to easily tell if fact values have changed.
I dunno, I think you've brought about a 4th dimension fear I've had about this stuff anyway, but its not specifically related to the NULL. It does however related to the migration 25 issue we've talked about already. Hmm.