Clean-up old transaction IDs on the background worker. (#8544)

changelog.d/8544.feature

+Allow running background tasks in a separate worker process.

synapse/storage/databases/main/events_worker.py

@@ -137,7 +137,7 @@ class EventsWorkerStore(SQLBaseStore):
                     db_conn, "events", "stream_ordering", step=-1
                 )
 
-        if not hs.config.worker.worker_app:
+        if hs.config.run_background_tasks:
             # We periodically clean out old transaction ID mappings
             self._clock.looping_call(
                 run_as_background_process,