Sliding Sync: Fix `_bulk_get_max_event_pos(...)` being inefficient (#17728)

Fix `_bulk_get_max_event_pos(...)` being inefficient. It kept adding all
of the `batch_results` to the `results` over and over every time we
checked a single room in the batch.

I think we still ended up with the right answer before because we
accumulate `recheck_rooms` and actually recheck them to overwrite the
bad data we wrote to the `results` before.

Introduced in
https://github.com/element-hq/synapse/pull/17606/files#diff-cbd54e4b5a2a1646299d659a2d5884d6cb14e608efd2e1658e72b465bb66e31bR1481

changelog.d/17728.misc

+Fix `_bulk_get_max_event_pos` being inefficient.

synapse/storage/databases/main/stream.py

@@ -1584,7 +1584,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
                 )
             for room_id, stream_ordering in batch_results.items():
                 if stream_ordering <= now_token.stream:
-                    results.update(batch_results)
+                    results[room_id] = stream_ordering
                 else:
                     recheck_rooms.add(room_id)