Optimize adding NOT NULL fields with initial values.
Previously, adding a field with NOT NULL and an initial value (to

update existing table entries) on MySQL or PostgreSQL required three

ALTER TABLE statements:
1) One to add the column (as NULL, with the DEFAULT, in order to

   update the existing entries)

2) One to drop the default

3) One to set the column to NOT NULL
The first and third resulted in a table rebuild (at least on MySQL). The

second, fortunately, doesn't. Still, this meant that many table adds

would cause MySQL to do a dump of the table, a rebuild, and a load of

the table data, twice per added field.
We now combine the first and third ALTER TABLE. We add the field as

NOT NULL with the DEFAULT, which will update all existing entries as

well, and then we drop the DEFAULT. There's now only one table rebuild

per added field.

Unit tests pass for all supported databases.
Tested an upgrade from RB 1.7.x to 2.0, which has plenty of these sorts of

updates. It reduced the number of ALTER TABLEs by 7 (these were 7 that would

trigger a table rebuild), and the evolutions passed. Verified in the database

that the schema was correct and that existing entries had the correct

defaults.