[r=allenap, bac, gmb, julian-edwards, wallyworld][bug=905853, 905855,
906079] In buildmaster,
always shift into a read-write database transaction access mode before
updating PackageBuild statuses. Shift into read-write transactions in
appropriate places in TranslationTemplatesBuildBehavior. Ensure that all
lp.buildmaster tests to which it is relevant are running with
BuilddManagerTestFixture.