simplify content storage in packed packets

It's not necessary to store both the packetBuffer and the slice
containing the raw data in the packet.

packet_handler_map.go

@@ -218,7 +218,7 @@ func (h *packetHandlerMap) listen() {
 	defer close(h.listening)
 	for {
 		buffer := getPacketBuffer()
-		data := buffer.Slice
+		data := buffer.Data[:protocol.MaxReceivePacketSize]
 		// The packet size should not exceed protocol.MaxReceivePacketSize bytes
 		// If it does, we only read a truncated packet, which will then end up undecryptable
 		n, addr, err := h.conn.ReadFrom(data)