Nasty migration bug

[crspybits]

I've been struggling for a couple of days with a bug that has turned out to be a migration bug in iOSBasics.

These migrations can only do column additions (and possibly deletions). There can be problems if they do content changes. I've just run into a nasty bug bug with that. The problem came up when m2021_08_02 and m2021_08_07 both hadn't yet been done by clients. But m2021_08_02 at the time tried to fetch rows (in order to make changes). But the row -> model code made use of the m2021_08_07 change which hadn't yet been applied.