State machine local child root cache. (#9107)

* cache root for child api.

* minimal testing

* Reset cache on test 'set_root'.

* Update primitives/state-machine/src/trie_backend_essence.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update primitives/state-machine/src/trie_backend_essence.rs

* Update primitives/state-machine/src/trie_backend_essence.rs

* Renaming to 'reset_cache'.

* correct rust fmt

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/primitives/state-machine/src/trie_backend.rs

@@ -326,6 +326,22 @@ pub mod tests {
 				.unwrap(),
 			Some(vec![142u8]),
 		);
+		// Change cache entry to check that caching is active.
+		test_trie
+			.essence
+			.cache
+			.write()
+			.child_root
+			.entry(b"sub1".to_vec())
+			.and_modify(|value| {
+				*value = None;
+			});
+		assert_eq!(
+			test_trie
+				.child_storage(&ChildInfo::new_default(CHILD_KEY_1), b"value3")
+				.unwrap(),
+			None,
+		);
 	}
 
 	#[test]