Ensure the user ID is serialized in the payload instead of used as an instance name. (#9130)

changelog.d/9130.feature

+Add experimental support for handling and persistence of to-device messages to happen on worker processes.

synapse/handlers/devicemessage.py

@@ -163,7 +163,7 @@ class DeviceMessageHandler:
             await self.store.mark_remote_user_device_cache_as_stale(sender_user_id)
 
             # Immediately attempt a resync in the background
-            run_in_background(self._user_device_resync, sender_user_id)
+            run_in_background(self._user_device_resync, user_id=sender_user_id)
 
     async def send_device_message(
         self,