Fix service setup for non-validator nodes (#378)

* Fix service setup for non-validator nodes

* Apply suggestions

polkadot/network/src/gossip.rs

@@ -169,6 +169,16 @@ pub fn register_validator<O: KnownOracle + 'static>(
 	RegisteredMessageValidator { inner: validator as _ }
 }
 
+/// Register a gossip validator for a non-authority node.
+pub fn register_non_authority_validator(service: Arc<NetworkService>) {
+	service.with_gossip(|gossip, ctx|
+		gossip.register_validator(
+			ctx,
+			POLKADOT_ENGINE_ID,
+			Arc::new(substrate_network::consensus_gossip::DiscardAll)),
+	);
+}
+
 /// A registered message validator.
 ///
 /// Create this using `register_validator`.