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/collator-protocol/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2018"
 
 [dependencies]
 always-assert = "0.1.2"
+derive_more = "0.99.14"
 futures = "0.3.15"
 futures-timer = "3"
 thiserror = "1.0.26"
@@ -29,5 +30,6 @@ assert_matches = "1.4.0"
 sp-core = { git = "https://github.com/paritytech/substrate", branch = "master", features = ["std"] }
 sp-keyring = { git = "https://github.com/paritytech/substrate", branch = "master" }
 sc-network = { git = "https://github.com/paritytech/substrate", branch = "master" }
+parity-scale-codec = { version = "2.0.0", features = ["std"]  }
 
 polkadot-subsystem-testhelpers = { package = "polkadot-node-subsystem-test-helpers", path = "../../subsystem-test-helpers" }