[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>

cumulus/pallets/xcmp-queue/src/tests.rs

@@ -22,8 +22,7 @@ use XcmpMessageFormat::*;
 use codec::Input;
 use cumulus_primitives_core::{ParaId, XcmpMessageHandler};
 use frame_support::{
-	assert_err, assert_noop, assert_ok, assert_storage_noop, hypothetically,
-	traits::{Footprint, Hooks},
+	assert_err, assert_noop, assert_ok, assert_storage_noop, hypothetically, traits::Hooks,
 	StorageNoopGuard,
 };
 use mock::{new_test_ext, ParachainSystem, RuntimeOrigin as Origin, Test, XcmpQueue};
@@ -100,7 +99,7 @@ fn xcm_enqueueing_multiple_times_works() {
 }
 
 #[test]
-#[cfg_attr(debug_assertions, should_panic = "Defensive failure")]
+#[cfg_attr(debug_assertions, should_panic = "Could not enqueue XCMP messages.")]
 fn xcm_enqueueing_starts_dropping_on_overflow() {
 	new_test_ext().execute_with(|| {
 		let xcm = VersionedXcm::<Test>::from(Xcm::<Test>(vec![ClearOrigin]));
@@ -112,12 +111,6 @@ fn xcm_enqueueing_starts_dropping_on_overflow() {
 			repeat((1000.into(), 1, data.as_slice())).take(limit * 2),
 			Weight::MAX,
 		);
-		assert_eq!(EnqueuedMessages::get().len(), limit);
-		// The drop threshold for pages is 48, the others numbers dont really matter:
-		assert_eq!(
-			<Test as Config>::XcmpQueue::footprint(1000.into()),
-			QueueFootprint { storage: Footprint { count: 256, size: 768 }, pages: 48 }
-		);
 	})
 }