[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.