network: check height before requesting headers

Only request headers from the other peer if his height is bigger than
ours. Otherwise we routinely ask 0-height newcomers for some random headers
that they know nothing about.

pkg/network/server.go

@@ -284,6 +284,8 @@ func (s *Server) PeerCount() int {
 // startProtocol starts a long running background loop that interacts
 // every ProtoTickInterval with the peer.
 func (s *Server) startProtocol(p Peer) {
+	var err error
+
 	log.WithFields(log.Fields{
 		"addr":        p.RemoteAddr(),
 		"userAgent":   string(p.Version().UserAgent),
@@ -292,11 +294,13 @@ func (s *Server) startProtocol(p Peer) {
 	}).Info("started protocol")
 
 	s.discovery.RegisterGoodAddr(p.PeerAddr().String())
-	err := s.requestHeaders(p)
+	if s.chain.HeaderHeight() < p.Version().StartHeight {
+		err = s.requestHeaders(p)
 		if err != nil {
 			p.Disconnect(err)
 			return
 		}
+	}
 
 	timer := time.NewTimer(s.ProtoTickInterval)
 	for {