Add ability to run replication protocol over redis. (#7040)

This is configured via the `redis` config options.

stubs/txredisapi.pyi

@@ -0,0 +1,40 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# 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.
+
+"""Contains *incomplete* type hints for txredisapi.
+"""
+
+from typing import List, Optional, Union
+
+class RedisProtocol:
+    def publish(self, channel: str, message: bytes): ...
+
+class SubscriberProtocol:
+    def subscribe(self, channels: Union[str, List[str]]): ...
+
+def lazyConnection(
+    host: str = ...,
+    port: int = ...,
+    dbid: Optional[int] = ...,
+    reconnect: bool = ...,
+    charset: str = ...,
+    password: Optional[str] = ...,
+    connectTimeout: Optional[int] = ...,
+    replyTimeout: Optional[int] = ...,
+    convertNumbers: bool = ...,
+) -> RedisProtocol: ...
+
+class SubscriberFactory:
+    def buildProtocol(self, addr): ...