Add send message events for gossip_queries

This change enables initiating gossip queries with a peer using the
SendMessageEvent enum. Specifically we add an event for sending
query_channel_range to discover the existance of channels and an event
for sending query_short_channel_ids to request routing gossip messages
for a set of channels. These events are handled inside the process_events
method of PeerManager which sends the serialized message to the peer.

Author: bmancini55

lightning/src/ln/channelmanager.rs

@@ -3404,6 +3404,8 @@ impl<ChanSigner: ChannelKeys, M: Deref + Sync + Send, T: Deref + Sync + Send, K:
 					&events::MessageSendEvent::BroadcastChannelUpdate { .. } => true,
 					&events::MessageSendEvent::HandleError { ref node_id, .. } => node_id != counterparty_node_id,
 					&events::MessageSendEvent::PaymentFailureNetworkUpdate { .. } => true,
+					&events::MessageSendEvent::SendChannelRangeQuery { .. } => false,
+					&events::MessageSendEvent::SendShortIdsQuery { .. } => false,
 				}
 			});
 		}

lightning/src/ln/peer_handler.rs

@@ -1131,6 +1131,16 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, L: Deref> PeerManager<D
 								self.do_attempt_write_data(&mut descriptor, peer);
 							},
 						}
+					},
+					MessageSendEvent::SendChannelRangeQuery { ref node_id, ref msg } => {
+						let (mut descriptor, peer) = get_peer_for_forwarding!(node_id, {});
+						peer.pending_outbound_buffer.push_back(peer.channel_encryptor.encrypt_message(&encode_msg!(msg)));
+						self.do_attempt_write_data(&mut descriptor, peer);
+					},
+					MessageSendEvent::SendShortIdsQuery { ref node_id, ref msg } => {
+						let (mut descriptor, peer) = get_peer_for_forwarding!(node_id, {});
+						peer.pending_outbound_buffer.push_back(peer.channel_encryptor.encrypt_message(&encode_msg!(msg)));
+						self.do_attempt_write_data(&mut descriptor, peer);
 					}
 				}
 			}

lightning/src/util/events.rs

@@ -346,7 +346,22 @@ pub enum MessageSendEvent {
 	PaymentFailureNetworkUpdate {
 		/// The channel/node update which should be sent to NetGraphMsgHandler
 		update: msgs::HTLCFailChannelUpdate,
-	}
+	},
+	/// Query a peer for channels with funding transaction UTXOs in a block range.
+	SendChannelRangeQuery {
+		/// The node_id of the node which should receive this message.
+		node_id: PublicKey,
+		/// The query_channel_range which should be sent.
+		msg: msgs::QueryChannelRange,
+	},
+	/// Request routing gossip messages from a peer for a list of channels identified by
+	/// their short_channel_ids.
+	SendShortIdsQuery {
+		/// The node_id of the node which should receive this message.
+		node_id: PublicKey,
+		/// The query_short_channel_ids which should be sent.
+		msg: msgs::QueryShortChannelIds,
+	},
 }
 
 /// A trait indicating an object may generate message send events