See code comments. The problem is that since I always write the
newest upgrader and use it immediately, I've never actually taken
a very old database and run it through the whole series of
upgraders. So that will be necessary to have more confidence in this
system.
There was always some semblance that two blank lines has some kind of
meaning or structure that's different from single blank lines, but
in reality it was mostly arbitrary and I can't stand to look at it
any more.
It turns out you can't adjust foreign_keys while a transaction is active,
so the upgraders that need to disable and re-enable them were not
working right and encountering foreign key violations.
