use ackhandler.Frame directly, not as a pointer, remove its sync.Pool (#3835)

internal/ackhandler/packet.go

@@ -12,7 +12,7 @@ type Packet struct {
 	SendTime        time.Time
 	PacketNumber    protocol.PacketNumber
 	StreamFrames    []StreamFrame
-	Frames          []*Frame
+	Frames          []Frame
 	LargestAcked    protocol.PacketNumber // InvalidPacketNumber if the packet doesn't contain an ACK
 	Length          protocol.ByteCount
 	EncryptionLevel protocol.EncryptionLevel
@@ -49,9 +49,6 @@ func GetPacket() *Packet {
 // We currently only return Packets back into the pool when they're acknowledged (not when they're lost).
 // This simplifies the code, and gives the vast majority of the performance benefit we can gain from using the pool.
 func putPacket(p *Packet) {
-	for _, f := range p.Frames {
-		putFrame(f)
-	}
 	p.Frames = nil
 	p.StreamFrames = nil
 	packetPool.Put(p)