no-txn, vacuum-only migrations

[mzabani]

Study the possibility of adding a new "vacuum-only" migration modifier that:

1 - Ensures DB checksums do not change and data is not deleted/inserted/updated 2 - Allows pre-commit checksum verifications to run for contiguous migration blocks of type [no-txn+vacuum-only,in-txn, no-txn+data-only]

The use would be for things such as VACUUM. What other statements would it be useful for? Maybe this is too specific and not worth it..