Fix query performance for /sync

[erikjohnston]

Broke in #12729.

[erikjohnston]

Old query plan:

                                                                QUERY PLAN                                                                
------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=10249770.65..10249770.66 rows=1 width=54)
   ->  Sort  (cost=10249770.65..10249770.66 rows=1 width=54)
         Sort Key: events.stream_ordering DESC
         ->  Hash Left Join  (cost=169036.84..10249770.64 rows=1 width=54)
               Hash Cond: (events.event_id = rejections.event_id)
               Filter: (rejections.reason IS NULL)
               ->  Bitmap Heap Scan on events  (cost=164761.43..10230672.09 rows=5646913 width=54)
                     Recheck Cond: ((room_id = '....'::text) AND (stream_ordering <= '2945535006'::bigint))
                     Filter: (NOT outlier)
                     ->  Bitmap Index Scan on events_room_stream2  (cost=0.00..163349.70 rows=5671300 width=0)
                           Index Cond: ((room_id = '...'::text) AND (stream_ordering <= '2945535006'::bigint))
               ->  Hash  (cost=2757.96..2757.96 rows=121396 width=52)
                     ->  Seq Scan on rejections  (cost=0.00..2757.96 rows=121396 width=52)
(13 rows)

new query plan:

 Limit  (cost=1.12..4.82 rows=1 width=54)
   ->  Nested Loop Anti Join  (cost=1.12..20857359.91 rows=5646697 width=54)
         ->  Index Scan Backward using events_room_stream2 on events  (cost=0.70..11327215.26 rows=5646916 width=54)
               Index Cond: ((room_id = '...'::text) AND (stream_ordering <= '2945535006'::bigint))
               Filter: (NOT outlier)
         ->  Index Only Scan using rejections_event_id_key on rejections  (cost=0.42..1.69 rows=1 width=41)
               Index Cond: (event_id = events.event_id)

[DMRobertson]

retargeted to release branch

[DMRobertson]

This was tested on matrix.org and made a huge improvement.