don't mark handshake packets as "lost" (#331)

Packet 1 is always a stage 1 handshake and packet 2 is always stage 2.
Normal packets don't start flowing until the message counter is 3 or
higher.

Currently we only receive either packet 1 or 2 depending on if
we are the initiator or responder for the handshake, so we end up
marking one of these as "lost". We should mark these packets as "seen"
when we are the one sending them, since we don't expect to see them from
the other side.

handshake_ix.go

@@ -63,6 +63,10 @@ func ixHandshakeStage0(f *Interface, vpnIp uint32, hostinfo *HostInfo) {
 		return
 	}
 
+	// We are sending handshake packet 1, so we don't expect to receive
+	// handshake packet 1 from the responder
+	ci.window.Update(1)
+
 	hostinfo.HandshakePacket[0] = msg
 	hostinfo.HandshakeReady = true
 	hostinfo.handshakeStart = time.Now()
@@ -198,6 +202,10 @@ func ixHandshakeStage1(f *Interface, addr *udpAddr, hostinfo *HostInfo, packet [
 			hostinfo.HandshakePacket[2] = make([]byte, len(msg))
 			copy(hostinfo.HandshakePacket[2], msg)
 
+			// We are sending handshake packet 2, so we don't expect to receive
+			// handshake packet 2 from the initiator.
+			ci.window.Update(2)
+
 			f.messageMetrics.Tx(handshake, NebulaMessageSubType(msg[1]), 1)
 			err := f.outside.WriteTo(msg, addr)
 			if err != nil {