[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.

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Francisco Aguirre <[email protected]>