Availability recovery subsystem (#2122)

* Adds message types

* Add code skeleton

* Adds subsystem code.

* Adds a first test

* Adds interaction result to availability_lru

* Use LruCache instead of a HashMap

* Whitespaces to tabs

* Do not ignore errors

* Change error type

* Add a timeout to chunk requests

* Add custom errors and log them

* Adds replace_availability_recovery method

* recovery_threshold computed by erasure crate

* change core to std

* adds docs to error type

* Adds a test for invalid reconstruction

* refactors interaction run into multiple methods

* Cleanup AwaitedChunks

* Even more fixes

* Test that recovery with wrong root is an error

* Break to launch another requests

* Styling fixes

* Add SessionIndex to API

* Proper relay parents for MakeRequest

* Remove validator_discovery and use message

* Remove a stream on exhaustion

* On cleanup free the request streams

* Fix merge and refactor

polkadot/node/subsystem/src/errors.rs

@@ -59,3 +59,21 @@ impl core::fmt::Display for ChainApiError {
 }
 
 impl std::error::Error for ChainApiError {}
+
+/// An error that may happen during Availability Recovery process.
+#[derive(PartialEq, Debug, Clone)]
+pub enum RecoveryError {
+	/// A chunk is recovered but is invalid.
+	Invalid,
+
+	/// A requested chunk is unavailable.
+	Unavailable,
+}
+
+impl std::fmt::Display for RecoveryError {
+	fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
+		write!(f, "{}", self)
+	}
+}
+
+impl std::error::Error for RecoveryError {}