keep track of which packets were sent as retransmissions

When an ACK for a packet that was retransmitted arrives, we don't need
to retransmit the retransmission, even if it was lost.

internal/ackhandler/interfaces.go

@@ -33,7 +33,7 @@ type SentPacketHandler interface {
 	GetPacketNumberLen(protocol.PacketNumber) protocol.PacketNumberLen
 
 	GetAlarmTimeout() time.Time
-	OnAlarm()
+	OnAlarm() error
 }
 
 // ReceivedPacketHandler handles ACKs needed to send for incoming packets