Force serialised creation of events per room

The default is 5, which intentionally allows for forks in the room
DAGs at the cost of dummy events and expensive state resolution. This
actually causes more problems in large, busy rooms.

synapse/handlers/message.py

@@ -497,7 +497,9 @@ def __init__(self, hs: "HomeServer"):
 
         # We arbitrarily limit concurrent event creation for a room to 5.
         # This is to stop us from diverging history *too* much.
-        self.limiter = Linearizer(max_count=5, name="room_event_creation_limit")
+        # Beeper: limit changed from 5 -> 1, see:
+        # https://mau.dev/maunium/synapse/-/commit/d4ec0b6762852ce2e7e9a741db57ad459922c5a3
+        self.limiter = Linearizer(max_count=1, name="room_event_creation_limit")
 
         self._bulk_push_rule_evaluator = hs.get_bulk_push_rule_evaluator()