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/lib.rs

@@ -74,8 +74,6 @@ pub use metered_channel as metered;
 pub use polkadot_node_network_protocol::MIN_GOSSIP_PEERS;
 
 pub use determine_new_blocks::determine_new_blocks;
-/// Error classification.
-pub use error_handling::{unwrap_non_fatal, Fault};
 
 /// These reexports are required so that external crates can use the `delegated_subsystem` macro properly.
 pub mod reexports {
@@ -88,7 +86,6 @@ pub mod rolling_session_window;
 pub mod runtime;
 
 mod determine_new_blocks;
-mod error_handling;
 
 #[cfg(test)]
 mod tests;