Fix thread BG update to not seq scan event_json (#11192)

For some reason the query optimiser decided to seq scan both tables,
rather than index scanning `event_json`.

changelog.d/11192.feature

@@ -0,0 +1 @@
+Experimental support for the thread relation defined in [MSC3440](https://github.com/matrix-org/matrix-doc/pull/3440).

synapse/storage/databases/main/events_bg_updates.py

@@ -1108,7 +1108,7 @@ class EventsBackgroundUpdatesStore(SQLBaseStore):
                 """
                 SELECT event_id, json FROM event_json
                 LEFT JOIN event_relations USING (event_id)
-                WHERE event_id > ? AND relates_to_id IS NULL
+                WHERE event_id > ? AND event_relations.event_id IS NULL
                 ORDER BY event_id LIMIT ?
                 """,
                 (last_event_id, batch_size),