Stub out ServerNoticesSender on the workers

... and have the sync endpoints call it directly rather than obsure indirection
via PresenceHandler

synapse/server_notices/worker_server_notices_sender.py

@@ -0,0 +1,46 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from twisted.internet import defer
+
+
+class WorkerServerNoticesSender(object):
+    """Stub impl of ServerNoticesSender which does nothing"""
+    def __init__(self, hs):
+        """
+        Args:
+            hs (synapse.server.HomeServer):
+        """
+
+    def on_user_syncing(self, user_id):
+        """Called when the user performs a sync operation.
+
+        Args:
+            user_id (str): mxid of user who synced
+
+        Returns:
+            Deferred
+        """
+        return defer.succeed()
+
+    def on_user_ip(self, user_id):
+        """Called on the master when a worker process saw a client request.
+
+        Args:
+            user_id (str): mxid
+
+        Returns:
+            Deferred
+        """
+        raise AssertionError("on_user_ip unexpectedly called on worker")