[SCHEMA] Migration for easeventuid.

Summary:
Note this does not include a script to backfill the table
for existing events.

Also note this does not include the msg_uid index because we don't need it
(will update code too).

Test Plan: Run it, visually inspect.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D874