New relayer rewards scheme integration (#1652)

* relayer rewards integration: initial commit

* added refund-relayer-extension to the millau runtime

* spelling

* spelling again

* new -> Default

bridges/primitives/messages/src/target_chain.rs

@@ -16,7 +16,7 @@
 
 //! Primitives of messages module, that are used on the target chain.
 
-use crate::{LaneId, Message, MessageKey, MessagePayload, OutboundLaneData};
+use crate::{LaneId, Message, MessageKey, MessageNonce, MessagePayload, OutboundLaneData};
 
 use bp_runtime::{messages::MessageDispatchResult, Size};
 use codec::{Decode, Encode, Error as CodecError};
@@ -112,6 +112,25 @@ pub trait MessageDispatch<AccountId> {
 	) -> MessageDispatchResult<Self::DispatchLevelResult>;
 }
 
+/// Manages payments that are happening at the target chain during message delivery transaction.
+pub trait DeliveryPayments<AccountId> {
+	/// Error type.
+	type Error: Debug + Into<&'static str>;
+
+	/// Pay rewards for delivering messages to the given relayer.
+	///
+	/// This method is called during message delivery transaction which has been submitted
+	/// by the `relayer`. The transaction brings `total_messages` messages  but only
+	/// `valid_messages` have been accepted. The post-dispatch transaction weight is the
+	/// `actual_weight`.
+	fn pay_reward(
+		relayer: AccountId,
+		total_messages: MessageNonce,
+		valid_messages: MessageNonce,
+		actual_weight: Weight,
+	);
+}
+
 impl<Message> Default for ProvedLaneMessages<Message> {
 	fn default() -> Self {
 		ProvedLaneMessages { lane_state: None, messages: Vec::new() }
@@ -130,6 +149,19 @@ impl<DispatchPayload: Decode> From<MessagePayload> for DispatchMessageData<Dispa
 	}
 }
 
+impl<AccountId> DeliveryPayments<AccountId> for () {
+	type Error = &'static str;
+
+	fn pay_reward(
+		_relayer: AccountId,
+		_total_messages: MessageNonce,
+		_valid_messages: MessageNonce,
+		_actual_weight: Weight,
+	) {
+		// this implementation is not rewarding relayer at all
+	}
+}
+
 /// Structure that may be used in place of `SourceHeaderChain` and `MessageDispatch` on chains,
 /// where inbound messages are forbidden.
 pub struct ForbidInboundMessages;