ackhandler: avoid storing packet number in packet struct (#5312)

* ackhandler: optimize memory layout of packet struct

The packet number can be derived from the position that this packet is
stored at in the packets slice in the sent packet history. There is no
need to store the packet number, saving 8 bytes per packet.

* ackhandler: avoid copying the packet struct

internal/ackhandler/packet.go

@@ -7,10 +7,14 @@ import (
 	"github.com/quic-go/quic-go/internal/protocol"
 )
 
+type packetWithPacketNumber struct {
+	PacketNumber protocol.PacketNumber
+	*packet
+}
+
 // A Packet is a packet
 type packet struct {
 	SendTime        time.Time
-	PacketNumber    protocol.PacketNumber
 	StreamFrames    []StreamFrame
 	Frames          []Frame
 	LargestAcked    protocol.PacketNumber // InvalidPacketNumber if the packet doesn't contain an ACK
@@ -33,7 +37,6 @@ var packetPool = sync.Pool{New: func() any { return &packet{} }}
 
 func getPacket() *packet {
 	p := packetPool.Get().(*packet)
-	p.PacketNumber = 0
 	p.StreamFrames = nil
 	p.Frames = nil
 	p.LargestAcked = 0