Timestamped version numbers don't get properly put in the migrate_versions table

[Andrew]

The code that updates the version table infers the "next" version number by taking the current version number and adding `self.step`. This does not work for timestamped version numbers (unless they are all 1 ms apart from each other).

I propose that we zip the versions with the changes corresponding to them when we create the changeset, and then get the version numbers that way.