Use changes tries in query_storage RPC (#1082)

* use changes tries in query_storage RPC

* let + match + return + call -> match

substrate/core/client/src/backend.rs

@@ -17,6 +17,7 @@
 //! Substrate Client data backend
 
 use crate::error;
+use primitives::ChangesTrieConfiguration;
 use runtime_primitives::{generic::BlockId, Justification, StorageMap, ChildrenStorageMap};
 use runtime_primitives::traits::{AuthorityIdFor, Block as BlockT, NumberFor};
 use state_machine::backend::Backend as StateBackend;
@@ -113,7 +114,7 @@ pub trait Backend<Block, H>: AuxStore + Send + Sync where
 	/// Associated state backend type.
 	type State: StateBackend<H>;
 	/// Changes trie storage.
-	type ChangesTrieStorage: StateChangesTrieStorage<H>;
+	type ChangesTrieStorage: PrunableStateChangesTrieStorage<H>;
 
 	/// Begin a new block insertion transaction with given parent block id.
 	/// When constructing the genesis, this is called with all-zero hash.
@@ -154,6 +155,12 @@ pub trait Backend<Block, H>: AuxStore + Send + Sync where
 	}
 }
 
+/// Changes trie storage that supports pruning.
+pub trait PrunableStateChangesTrieStorage<H: Hasher>: StateChangesTrieStorage<H> {
+	/// Get number block of oldest, non-pruned changes trie.
+	fn oldest_changes_trie_block(&self, config: &ChangesTrieConfiguration, best_finalized: u64) -> u64;
+}
+
 /// Mark for all Backend implementations, that are making use of state data, stored locally.
 pub trait LocalBackend<Block, H>: Backend<Block, H>
 where