Define DEFAULT_PARSE_LAZY, DEFAULT_PARSE_RETAIN so that we can use them on subclasses of Message.
Why? Subclasses of Message class need to know the default value of parse_lazy and parse_retain, because in Scala, we can call only one kind of super constructor. If we could call different kinds of constructors on Message, we don't need to know the default value of these fields. As a solution to provide different auxiliary constructors on subclasses of Message class, we call most general constructor of Message class with default values of each field.