[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/Cargo.toml

@@ -13,6 +13,7 @@ codec = { package = "parity-scale-codec", version = "3.6.1", default-features =
 scale-info = { version = "2.10.0", default-features = false, features = ["derive"] }
 serde = { version = "1.0.193", optional = true, features = ["derive"] }
 log = { version = "0.4.17", default-features = false }
+environmental = { version = "1.1.4", default-features = false }
 
 sp-core = { path = "../../primitives/core", default-features = false }
 sp-io = { path = "../../primitives/io", default-features = false }
@@ -34,6 +35,7 @@ rand_distr = "0.4.3"
 default = ["std"]
 std = [
 	"codec/std",
+	"environmental/std",
 	"frame-benchmarking?/std",
 	"frame-support/std",
 	"frame-system/std",