discovery/syncer: make gossip sends synchronous

This commit makes all replies in the gossip syncer synchronous, meaning
that they will wait for each message to be successfully written to the
remote peer before attempting to send the next. This helps throttle
messages the remote peer has requested, preventing unintended
disconnects when the remote peer is slow to process messages. This
changes also helps out congestion in the peer by forcing the syncer to
buffer the messages instead of dumping them into the peer's queue.

discovery/sync_manager.go

@@ -386,6 +386,9 @@ func (m *SyncManager) createGossipSyncer(peer lnpeer.Peer) *GossipSyncer {
 		sendToPeer: func(msgs ...lnwire.Message) error {
 			return peer.SendMessageLazy(false, msgs...)
 		},
+		sendToPeerSync: func(msgs ...lnwire.Message) error {
+			return peer.SendMessageLazy(true, msgs...)
+		},
 	})
 
 	// Gossip syncers are initialized by default in a PassiveSync type