implement sending of shortened packet numbers

ackhandler/interfaces.go

@@ -15,6 +15,7 @@ type SentPacketHandler interface {
 	DequeuePacketForRetransmission() (packet *Packet)
 
 	BytesInFlight() protocol.ByteCount
+	GetLargestObserved() protocol.PacketNumber
 }
 
 // ReceivedPacketHandler handles ACKs needed to send for incoming packets

ackhandler/sent_packet_handler.go

@@ -220,3 +220,7 @@ func (h *sentPacketHandler) DequeuePacketForRetransmission() (packet *Packet) {
 func (h *sentPacketHandler) BytesInFlight() protocol.ByteCount {
 	return h.bytesInFlight
 }
+
+func (h *sentPacketHandler) GetLargestObserved() protocol.PacketNumber {
+	return h.LargestObserved
+}