Add append api and use it to deposit events (#5795)

* use append api to deposit events

* use optimized append

* one more optimization

* fix bug

* fix issues

* address review

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

@@ -223,6 +223,13 @@ impl OverlayedChanges {
 			})
 	}
 
+	/// Returns mutable reference to either commited or propsective value.
+	pub fn value_mut(&mut self, key: &[u8]) -> Option<&mut Option<StorageValue>> {
+		// not using map because of double borrow inside closure
+		if let Some(entry) = self.prospective.top.get_mut(key) { return Some(&mut entry.value) }
+		return self.committed.top.get_mut(key).map(|entry| &mut entry.value);
+	}
+
 	/// Returns a double-Option: None if the key is unknown (i.e. and the query should be referred
 	/// to the backend); Some(None) if the key has been deleted. Some(Some(...)) for a key whose
 	/// value has been set.