Issue #147000 by Berdir, Dave Reid, pwolanin, pillarsdotnet, TR, jbrauer,...

Issue #147000 by Berdir, Dave Reid, pwolanin, pillarsdotnet, TR, jbrauer, dstol: rework module_rebuild_cache() and system_theme_data() to support locking, so that concurrent requests will not corrupt the data; fixes an issue where themes got disabled occasionally