increase legacy highestInOrderObserved properly with reordering

fixes #237

ackhandlerlegacy/received_packet_handler.go

@@ -68,16 +68,21 @@ func (h *receivedPacketHandler) ReceivedPacket(packetNumber protocol.PacketNumbe
 		h.largestObserved = packetNumber
 	}
 
-	if packetNumber == h.highestInOrderObserved+1 {
-		h.highestInOrderObserved = packetNumber
-		h.highestInOrderObservedEntropy.Add(packetNumber, entropyBit)
-	}
-
 	h.packetHistory[packetNumber] = packetHistoryEntry{
 		EntropyBit:   entropyBit,
 		TimeReceived: time.Now(),
 	}
 
+	// Try to increase highestInOrderObserved as far as possible
+	for {
+		p, ok := h.packetHistory[h.highestInOrderObserved+1]
+		if !ok {
+			break
+		}
+		h.highestInOrderObserved++
+		h.highestInOrderObservedEntropy.Add(h.highestInOrderObserved, p.EntropyBit)
+	}
+
 	h.garbageCollect()
 
 	if uint32(len(h.packetHistory)) > protocol.MaxTrackedReceivedPackets {