Custom relay strategy (#1198)

* Add relayer strategy

* Add default relayer strategy

* default relayer strategy

* expose relayer strategy

* fix compile

* fix compile

* docs

* Rename Relayer to Relay, keep RelayerDecide

* split `DefaultRelayerStrategy` into `AltruisticRelayerStrategy` and `RationalRelayerStrategy`

* Remove relayer mode

* Remove unused import

* Rename `RelayerStrategy` to `RelayStrategy`

* Add missing docs

* clippy

* clippy

* clippy

* clippy

* Revert `relayer_mode` and add `MixStrategy`

* Add `EnforcementStrategy`

* fix bug and simplify relay strategy

* Update message_lane_loop.rs

* Update messages_target.rs

* clippy

* clippy

* clippy

* clippy

* clippy

* clippy

* clippy

* fix test

* fix test

* test

test

test

fix test

bridges/relays/messages/src/relay_strategy/altruistic_strategy.rs

@@ -0,0 +1,45 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Altruistic relay strategy
+
+use async_trait::async_trait;
+
+use crate::{
+	message_lane::MessageLane,
+	message_lane_loop::{
+		SourceClient as MessageLaneSourceClient, TargetClient as MessageLaneTargetClient,
+	},
+	relay_strategy::{RelayReference, RelayStrategy},
+};
+
+/// The relayer doesn't care about rewards.
+#[derive(Clone)]
+pub struct AltruisticStrategy;
+
+#[async_trait]
+impl RelayStrategy for AltruisticStrategy {
+	async fn decide<
+		P: MessageLane,
+		SourceClient: MessageLaneSourceClient<P>,
+		TargetClient: MessageLaneTargetClient<P>,
+	>(
+		&self,
+		_reference: &mut RelayReference<P, SourceClient, TargetClient>,
+	) -> bool {
+		true
+	}
+}