Attempt to include db threads in cpu usage stats (#3496)

Let's try to include time spent in the DB threads in the per-request/block cpu
usage metrics.

synapse/storage/events_worker.py

@@ -261,7 +261,8 @@ class EventsWorkerStore(SQLBaseStore):
                 ]
 
                 rows = self._new_transaction(
-                    conn, "do_fetch", [], [], None, self._fetch_event_rows, event_ids
+                    conn, "do_fetch", [], [],
+                    self._fetch_event_rows, event_ids,
                 )
 
                 row_dict = {