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/v1.rs

@@ -25,7 +25,7 @@ use polkadot_primitives::v1::{
 	CandidateHash, CandidateReceipt, CommittedCandidateReceipt, Hash, Id as ParaId, ValidatorIndex,
 };
 
-use super::{request::IsRequest, Protocol};
+use super::{IsRequest, Protocol};
 
 /// Request an availability chunk.
 #[derive(Debug, Copy, Clone, Encode, Decode)]