Fast sync child trie support. (#9239)

* state machine proofs.

* initial implementation

* Remove todo.

* Extend test and fix import.

* fix no proof, with proof ko.

* fix start at logic.

* Restore response size.

* Rework comments.

* Add explicit ref

* Use compact proof.

* ref change

* elaborato on empty change set condition.

* KeyValueState renaming.

* Do not add two time child trie with same root to sync reply.

* rust format

* Fix merge.

* fix warnings and fmt

* fmt

* update protocol id to V2

substrate/client/consensus/common/src/block_import.rs

@@ -133,7 +133,7 @@ pub struct ImportedState<B: BlockT> {
 	/// Target block hash.
 	pub block: B::Hash,
 	/// State keys and values.
-	pub state: Vec<(Vec<u8>, Vec<u8>)>,
+	pub state: sp_state_machine::KeyValueStates,
 }
 
 impl<B: BlockT> std::fmt::Debug for ImportedState<B> {