Downward Message Processing implementation (#1859)

* DMP: data structures and plumbing

* DMP: Implement DMP logic in the router module

DMP: Integrate DMP parts into the inclusion module

* DMP: Introduce the max size limit for the size of a downward message

* DMP: Runtime API for accessing inbound messages

* OCD

Small clean ups

* DMP: fix the naming of the error

* DMP: add caution about a non-existent recipient

polkadot/runtime/parachains/src/runtime_api_impl/v1.rs

@@ -23,10 +23,11 @@ use primitives::v1::{
 	Id as ParaId, OccupiedCoreAssumption, SessionIndex, ValidationCode,
 	CommittedCandidateReceipt, ScheduledCore, OccupiedCore, CoreOccupied, CoreIndex,
 	GroupIndex, CandidateEvent, PersistedValidationData, AuthorityDiscoveryId,
+	InboundDownwardMessage,
 };
 use sp_runtime::traits::Zero;
 use frame_support::debug;
-use crate::{initializer, inclusion, scheduler, configuration, paras};
+use crate::{initializer, inclusion, scheduler, configuration, paras, router};
 
 /// Implementation for the `validators` function of the runtime API.
 pub fn validators<T: initializer::Trait>() -> Vec<ValidatorId> {
@@ -299,3 +300,10 @@ where
 		validator_index.and_then(|i| authorities.get(i).cloned())
 	}).collect()
 }
+
+/// Implementation for the `dmq_contents` function of the runtime API.
+pub fn dmq_contents<T: router::Trait>(
+	recipient: ParaId,
+) -> Vec<InboundDownwardMessage<T::BlockNumber>> {
+	<router::Module<T>>::dmq_contents(recipient)
+}