[FRAME] Make MQ pallet re-entrancy safe (#2356)

Closes https://github.com/paritytech/polkadot-sdk/issues/2319

Changes:
- Ensure that only `enqueue_message(s)` is callable from within the
message processor. This prevents messed up storage that can currently
happen when the pallet is called into recursively.
- Use `H256` instead of `[u8; 32]` for clearer API.

## Details

The re-entracy check is done with the `environmental` crate by adding a
`with_service_mutex(f)` function that runs the closure exclusively. This
works since the MQ pallet is not instantiable.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>

polkadot/runtime/parachains/src/ump_tests.rs

@@ -523,21 +523,21 @@ fn overweight_queue_works() {
 		assert_last_events(
 			[
 				pallet_message_queue::Event::<Test>::Processed {
-					id: hash_1,
+					id: hash_1.into(),
 					origin: Ump(UmpQueueId::Para(para_a)),
 					weight_used: Weight::from_parts(301, 301),
 					success: true,
 				}
 				.into(),
 				pallet_message_queue::Event::<Test>::OverweightEnqueued {
-					id: hash_2,
+					id: hash_2.into(),
 					origin: Ump(UmpQueueId::Para(para_a)),
 					page_index: 0,
 					message_index: 1,
 				}
 				.into(),
 				pallet_message_queue::Event::<Test>::OverweightEnqueued {
-					id: hash_3,
+					id: hash_3.into(),
 					origin: Ump(UmpQueueId::Para(para_a)),
 					page_index: 0,
 					message_index: 2,
@@ -565,7 +565,7 @@ fn overweight_queue_works() {
 		));
 		assert_last_event(
 			pallet_message_queue::Event::<Test>::Processed {
-				id: hash_3,
+				id: hash_3.into(),
 				origin: Ump(UmpQueueId::Para(para_a)),
 				weight_used: Weight::from_parts(501, 501),
 				success: true,