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/erasure-coding/src/lib.rs

@@ -119,6 +119,7 @@ impl CodeParams {
 			.expect("this struct is not created with invalid shard number; qed")
 	}
 }
+
 /// Returns the maximum number of allowed, faulty chunks
 /// which does not prevent recovery given all other pieces
 /// are correct.