Allow UNION queries to be detected as reads for CRSF purposes

Summary: Currently, we don't correctly detect queries in the form `(SELECT ...) UNION (SELECT ...)` as reads. Detect them.

Test Plan: No more CSRFs when doing UNIONs.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D6342