Consolidate ALTER statements for migration performance.

Summary:
Doing multiple ALTERs separately may result in multiple table copies. This
modification makes this migration run in ~3min on production. (Not sure what
it was before except for "longer".)

Test Plan: comparison of emitted SQL with echo=True

Reviewers: charles

Reviewed By: charles

Differential Revision: https://review.inboxapp.com/D430