Remove request multiplexer (#3624)

* WIP: Get rid of request multiplexer.

* WIP

* Receiver for handling of incoming requests.

* Get rid of useless `Fault` abstraction.

The things the type system let us do are not worth getting abstracted in
its own type. Instead error handling is going to be merely a pattern.

* Make most things compile again.

* Port availability distribution away from request multiplexer.

* Formatting.

* Port dispute distribution over.

* Fixup statement distribution.

* Handle request directly in collator protocol.

+ Only allow fatal errors at top level.

* Use direct request channel for availability recovery.

* Finally get rid of request multiplexer

Fixes #2842 and paves the way for more back pressure possibilities.

* Fix overseer and statement distribution tests.

* Fix collator protocol and network bridge tests.

* Fix tests in availability recovery.

* Fix availability distribution tests.

* Fix dispute distribution tests.

* Add missing dependency

* Typos.

* Review remarks.

* More remarks.

polkadot/node/network/protocol/src/request_response/mod.rs

@@ -40,11 +40,14 @@ use strum::EnumIter;
 
 pub use sc_network::{config as network, config::RequestResponseConfig};
 
-/// All requests that can be sent to the network bridge.
-pub mod request;
-pub use request::{
-	IncomingRequest, OutgoingRequest, OutgoingResult, Recipient, Requests, ResponseSender,
-};
+/// Everything related to handling of incoming requests.
+pub mod incoming;
+/// Everything related to handling of outgoing requests.
+pub mod outgoing;
+
+pub use incoming::{IncomingRequest, IncomingRequestReceiver};
+
+pub use outgoing::{OutgoingRequest, OutgoingResult, Recipient, Requests, ResponseSender};
 
 ///// Multiplexer for incoming requests.
 // pub mod multiplexer;
@@ -248,3 +251,12 @@ impl Protocol {
 		}
 	}
 }
+
+/// Common properties of any `Request`.
+pub trait IsRequest {
+	/// Each request has a corresponding `Response`.
+	type Response;
+
+	/// What protocol this `Request` implements.
+	const PROTOCOL: Protocol;
+}