Split different `RDATA` streams into different redis channels

[erikjohnston]

Similar to #12460, we should have a separate pub/sub channel for each RDATA stream so that workers only need to subscribe to the channels they actually need.

This is a bit tricky to do safely, i.e. that workers do correctly subscribe to streams they need, especially for caching purposes. I think the best approach is instead of having the replication client call into the various handlers and storage classes:

synapse/synapse/replication/tcp/client.py

Lines 134 to 148 in e3a49f4

	async def on_rdata(
	self, stream_name: str, instance_name: str, token: int, rows: list
	) -> None:
	"""Called to handle a batch of replication data with a given stream token.
	By default this just pokes the slave store. Can be overridden in subclasses to
	handle more.
	Args:
	stream_name: name of the replication stream for this batch of rows
	instance_name: the instance that wrote the rows.
	token: stream token for this batch of rows
	rows: a list of Stream.ROW_TYPE objects as returned by Stream.parse_row.
	"""
	self.store.process_replication_rows(stream_name, instance_name, token, rows)

instead each class registers with the replication client in their __init__ each stream they care about with a callback to handle that particular streams rows. That way a worker only subscribes to a stream if a class that requires is actually instantiated.

The downside of this is that it's a bit magic, and pulling in a class can have the side effect of causing a stream to suddenly be subscribed to.