Use "w" transactions for reading if they exist, so we can do transactional reads

Summary: Generally moves us toward having a sane approach to transaction handling.

Test Plan: See test case, which fails before this patch and passes afterwards.

Reviewers: vrana, btrahan, jungejason

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2394