`merge.resolveDefaults` is extremely slow

I don't have a profile, but it was causing a customer's 9 second operation to take hours because we were running it for every row in the diff.

We no longer run it for every row in the diff, but it's still slow because it works by calling `parseCreateTable`, which generates a `CREATE TABLE` string from the schema, and then parses it in yacc. This feels extremely unnecessary. We should be able to get the data from the schema directly.