[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>

substrate/frame/message-queue/src/mock_helpers.rs

@@ -15,6 +15,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#![allow(missing_docs)]
+
 //! Std setup helpers for testing and benchmarking.
 //!
 //! Cannot be put into mock.rs since benchmarks require no-std and mock.rs is std.
@@ -88,10 +90,12 @@ pub fn page<T: Config>(msg: &[u8]) -> PageOf<T> {
 	PageOf::<T>::from_message::<T>(msg.try_into().unwrap())
 }
 
+/// Create a book with a single message of one byte.
 pub fn single_page_book<T: Config>() -> BookStateOf<T> {
 	BookState { begin: 0, end: 1, count: 1, message_count: 1, size: 1, ..Default::default() }
 }
 
+/// Create an empty book.
 pub fn empty_book<T: Config>() -> BookStateOf<T> {
 	BookState { begin: 0, end: 1, count: 1, ..Default::default() }
 }