Changes tries build cache (#2933)

* changes tries build cache

added CT build cache test

* fix lines width

* fixed some grumbles

* clear cache when: digests disabled, top-level or skewed digest is built

* cached_changed_keys -> with_cached_changed_keys

substrate/core/client/src/client.rs

@@ -43,7 +43,8 @@ use sr_primitives::{
 use state_machine::{
 	DBValue, Backend as StateBackend, CodeExecutor, ChangesTrieAnchorBlockId,
 	ExecutionStrategy, ExecutionManager, prove_read, prove_child_read,
-	ChangesTrieRootsStorage, ChangesTrieStorage, ChangesTrieConfigurationRange,
+	ChangesTrieRootsStorage, ChangesTrieStorage,
+	ChangesTrieTransaction, ChangesTrieConfigurationRange,
 	key_changes, key_changes_proof, OverlayedChanges, NeverOffchainExt,
 };
 use executor::{RuntimeVersion, RuntimeInfo};
@@ -86,7 +87,7 @@ type StorageUpdate<B, Block> = <
 		<B as backend::Backend<Block, Blake2Hasher>>::BlockImportOperation
 			as BlockImportOperation<Block, Blake2Hasher>
 	>::State as state_machine::Backend<Blake2Hasher>>::Transaction;
-type ChangesUpdate = trie::MemoryDB<Blake2Hasher>;
+type ChangesUpdate<Block> = ChangesTrieTransaction<Blake2Hasher, NumberFor<Block>>;
 
 /// Execution strategies settings.
 #[derive(Debug, Clone)]
@@ -635,6 +636,14 @@ impl<B, E, Block, RA> Client<B, E, Block, RA> where
 				self
 			}
 
+			fn with_cached_changed_keys(
+				&self,
+				root: &H256,
+				functor: &mut dyn FnMut(&HashMap<Option<Vec<u8>>, HashSet<Vec<u8>>>),
+			) -> bool {
+				self.storage.with_cached_changed_keys(root, functor)
+			}
+
 			fn get(&self, key: &H256, prefix: Prefix) -> Result<Option<DBValue>, String> {
 				self.storage.get(key, prefix)
 			}
@@ -1001,7 +1010,7 @@ impl<B, E, Block, RA> Client<B, E, Block, RA> where
 		body: Option<Vec<Block::Extrinsic>>,
 	) -> error::Result<(
 		Option<StorageUpdate<B, Block>>,
-		Option<Option<ChangesUpdate>>,
+		Option<Option<ChangesUpdate<Block>>>,
 		Option<(
 			Vec<(Vec<u8>, Option<Vec<u8>>)>,
 			Vec<(Vec<u8>, Vec<(Vec<u8>, Option<Vec<u8>>)>)>