master doesn't send replication traffic unless you have a TCP replication listener

the TCP replication listener is unused if you have redis, so you'd expect to be able to remove it (indeed, the [docs](https://github.com/matrix-org/synapse/blob/85155654c52bb9999b6b4bcbcf84cd68505d4c82/docs/workers.md#experimental-support-for-replication-over-redis) say that you can). But if you do that, nothing builds the `ReplicationStreamer`, so no replication traffic ever gets sent.