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

This is configured via the `redis` config options.

synapse/config/redis.py

@@ -0,0 +1,35 @@
+# -*- 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.
+
+from synapse.config._base import Config
+from synapse.python_dependencies import check_requirements
+
+
+class RedisConfig(Config):
+    section = "redis"
+
+    def read_config(self, config, **kwargs):
+        redis_config = config.get("redis", {})
+        self.redis_enabled = redis_config.get("enabled", False)
+
+        if not self.redis_enabled:
+            return
+
+        check_requirements("redis")
+
+        self.redis_host = redis_config.get("host", "localhost")
+        self.redis_port = redis_config.get("port", 6379)
+        self.redis_dbid = redis_config.get("dbid")
+        self.redis_password = redis_config.get("password")