R1:c1f234759b04

Prefer MySQLi over MySQL

Summary: Makes sense with `QueryFuture`.

Test Plan: Switched secure.phabricator.com to MySQLi and nothing exploded.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5066