Add a new NodeFeatures constructor to capture the types of flags

When ChannelMessageHandler implementations wish to return a NodeFeatures which
contain all the known flags that are relevant to channel handling, but not
gossip  handling, they currently need to do so by manually constructing a
NodeFeatures with all known flags and then clearing the ones they don't want.

Instead of spreading this logic across the codebase, this consolidates such
construction into one place in features.rs.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -6122,7 +6122,7 @@ impl<Signer: Sign, M: Deref , T: Deref , K: Deref , F: Deref , L: Deref >
 	}
 
 	fn provided_node_features(&self) -> NodeFeatures {
-		NodeFeatures::known()
+		NodeFeatures::known_channel_features()
 	}
 
 	fn provided_init_features(&self, _their_init_features: &PublicKey) -> InitFeatures {

lightning/src/ln/features.rs

@@ -164,7 +164,8 @@ mod sealed {
 		],
 		optional_features: [
 			// Note that if new "non-channel-related" flags are added here they should be
-			// explicitly cleared in InitFeatures::known_channel_features.
+			// explicitly cleared in InitFeatures::known_channel_features and
+			// NodeFeatures::known_channel_features.
 			// Byte 0
 			DataLossProtect | InitialRoutingSync | UpfrontShutdownScript | GossipQueries,
 			// Byte 1
@@ -558,6 +559,14 @@ impl InitFeatures {
 	}
 }
 
+impl NodeFeatures {
+	/// Returns the set of known node features that are related to channels. At least some of
+	/// these features are likely required for peers to talk to us.
+	pub fn known_channel_features() -> NodeFeatures {
+		Self::known().clear_gossip_queries()
+	}
+}
+
 impl InvoiceFeatures {
 	/// Converts `InvoiceFeatures` to `Features<C>`. Only known `InvoiceFeatures` relevant to
 	/// context `C` are included in the result.

lightning/src/util/test_utils.rs

@@ -358,7 +358,7 @@ impl msgs::ChannelMessageHandler for TestChannelMessageHandler {
 		self.received_msg(wire::Message::Error(msg.clone()));
 	}
 	fn provided_node_features(&self) -> NodeFeatures {
-		NodeFeatures::empty()
+		NodeFeatures::known_channel_features()
 	}
 	fn provided_init_features(&self, _their_init_features: &PublicKey) -> InitFeatures {
 		InitFeatures::known_channel_features()