replace `tokio::sync::broadcast` for Pool's `ChannelManager` to `Downstream` sender

[plebhash]

similar to #215 but for this:

sv2-apps/pool-apps/pool/src/lib/mod.rs

Lines 66 to 67 in fd0ff79

	let (channel_manager_to_downstream_sender, _channel_manager_to_downstream_receiver) =
	broadcast::channel(10);