Support for selecting the Redis logical database. (#15034)

Note that this is only used for key-value store (cached values)
and not for the pub/sub replication used by Synapse.

synapse/server.py

@@ -827,6 +827,7 @@ class HomeServer(metaclass=abc.ABCMeta):
             hs=self,
             host=self.config.redis.redis_host,
             port=self.config.redis.redis_port,
+            dbid=self.config.redis.redis_dbid,
             password=self.config.redis.redis_password,
             reconnect=True,
         )