Load previous tags in ChangeTags::addTags() from DB_MASTER

This fixes a bug in mobile [1] that the tag added by MobileApp gets
overwritten by MobileFrontend, where both use ChangeTags::addTags(),
and MobileFrontend is registered (thus called) after MobileApp.

[1] http://lists.wikimedia.org/pipermail/mobile-l/2014-June/007260.html

Change-Id: Ie47649ef45f9eebdfeae73a0698ea5d21b150b56
(cherry picked from commit bc2ffb28d6ce667e6a6127e07e203a97150228c8)