A fast-path for requesting AvailableData from backing validators (#2453)

* guide changes for a fast-path requesting from backing validators

* add backing group to availability recovery message

* add new phase to interaction

* typos

* add full data messages

* handle new network messages

* dispatch full data requests

* cleanup

* check chunk index

* test for invalid recovery

* tests

* Typos.

* fix some grumbles

* be more explicit about error handling and control flow

* fast-path param

* use with_chunks_only in Service

Co-authored-by: Robert Klotzner <robert.klotzner@gmx.at>

polkadot/node/subsystem/src/messages.rs

@@ -41,7 +41,7 @@ use polkadot_primitives::v1::{
 	PersistedValidationData, PoV, SessionIndex, SignedAvailabilityBitfield,
 	ValidationCode, ValidatorId, CandidateHash,
 	ValidatorIndex, ValidatorSignature, InboundDownwardMessage, InboundHrmpMessage,
-	CandidateIndex,
+	CandidateIndex, GroupIndex,
 };
 use polkadot_statement_table::v1::Misbehavior;
 use std::{sync::Arc, collections::btree_map::BTreeMap};
@@ -282,6 +282,7 @@ pub enum AvailabilityRecoveryMessage {
 	RecoverAvailableData(
 		CandidateReceipt,
 		SessionIndex,
+		Option<GroupIndex>, // Optional backing group to request from first.
 		oneshot::Sender<Result<AvailableData, crate::errors::RecoveryError>>,
 	),
 	/// Event from the network bridge.