Add missing dm_update_lock

mautrix_amp/user.py

@@ -54,6 +54,7 @@ class User(DBUser, BaseUser):
         self.log = self.log.getChild(self.mxid)
         self._metric_value = defaultdict(lambda: False)
         self._connection_check_task = None
+        self.dm_update_lock = asyncio.Lock()
         self.client = None
         self.intent = None