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/subsystem-util/src/runtime/error.rs

@@ -23,23 +23,16 @@ use thiserror::Error;
 use polkadot_node_subsystem::errors::RuntimeApiError;
 use polkadot_primitives::v1::SessionIndex;
 
-use crate::Fault;
-
 pub type Result<T> = std::result::Result<T, Error>;
 
 /// Errors for `Runtime` cache.
-pub type Error = Fault<NonFatal, Fatal>;
-
-impl From<NonFatal> for Error {
-	fn from(e: NonFatal) -> Self {
-		Self::from_non_fatal(e)
-	}
-}
-
-impl From<Fatal> for Error {
-	fn from(f: Fatal) -> Self {
-		Self::from_fatal(f)
-	}
+#[derive(Debug, Error, derive_more::From)]
+#[error(transparent)]
+pub enum Error {
+	/// All fatal errors.
+	Fatal(Fatal),
+	/// All nonfatal/potentially recoverable errors.
+	NonFatal(NonFatal),
 }
 
 /// Fatal runtime errors.