Define virtual type_handler() for all Item classes

As of version 10.3.0, Item::type_handler is defined as:

virtual const Type_handler *type_handler() const

{

  return Type_handler::get_handler_by_field_type(field_type());

}

That is type handler is determined from field_type() by default.
This involves lookup inside get_handler_by_field_type(), which can affect performance negatively.
Also, this is not friendly for pluggable data types, which will define their own functions returning values of non-standard data types.

We'll change the definition of the method to:

virtual const Type_handler *type_handler() const= 0;

and implement it for all classes Item_xxx.