Alter Table might re-use column IDs after drop/add column

In further investigating various alter table bugs, I think I discovered another one: our Schema object doesn't maintain any "next_unused_column_id_" member or anything. So, if you issue an ALTER which drops a column, and then a separate ALTER which adds a new one, you may end up resuscitating old data. Even worse, it might be old data from an entirely unrelated column with a different type.

There's a test which attempts to show this doesn't happen in tablet-schema-test, but it's too localized to Tablet and reuses a single SchemaBuilder which does properly avoid column id reuse. We need an integration test for this case.