make SentPacketHandler compute the delta between sent and ack time

ackhandler/interfaces.go

@@ -1,6 +1,8 @@
 package ackhandler
 
 import (
+	"time"
+
 	"github.com/lucas-clemente/quic-go/frames"
 	"github.com/lucas-clemente/quic-go/protocol"
 )
@@ -8,7 +10,7 @@ import (
 // SentPacketHandler handles ACKs received for outgoing packets
 type SentPacketHandler interface {
 	SentPacket(packet *Packet) error
-	ReceivedAck(ackFrame *frames.AckFrame) error
+	ReceivedAck(ackFrame *frames.AckFrame) (time.Duration, error)
 
 	DequeuePacketForRetransmission() (packet *Packet)
 }