improve Notification.ToBinary() to reduce memory copy

```
var placeholder = [5]byte{}

func (n Notification) ToBinary() ([]byte, error) {
   ....
   /* write header place holder*/
   buf.Write(placeholder[:])

   /* write fields: token,payload... etc */
   ...

   /* rewrite header */
   b = buf.Bytes()
   b[0] = commandID
   binary.BigEndian.PutUint32(b[1:5], uint32(len(b)-5))
   return b, nil
}
```
