Set db isolation level to READ COMMITTED.

Summary:
Hopefully this will help resolve concurrency issues in the sync code in which
queries one one Greenlet weren't seeing database updates committed by another
Greenlet.

Test Plan:
Well... can no longer seem to reproduce the IntegrityErrors I was
getting.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D122