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/availability-distribution/src/pov_requester/mod.rs

@@ -19,7 +19,7 @@
 use futures::{channel::oneshot, future::BoxFuture, FutureExt};
 
 use polkadot_node_network_protocol::request_response::{
-	request::{RequestError, Requests},
+	outgoing::{RequestError, Requests},
 	v1::{PoVFetchingRequest, PoVFetchingResponse},
 	OutgoingRequest, Recipient,
 };