Split Item_type_holder::display_length into virtual methods in Type_handler

The method Item_type_holder::display_length(Item *) has this switch:

  switch (item->field_type())

This is not friendly to pluggable data types.

We'll introduce a new Type_handler virtual method:

virtual uint32 max_display_length(const Item *item) const;

and move the pieces from Item_type_holder::display_length(Item *) into implementations of various Type_handler_xxx::max_display_length().

Additionally, instead of doing cast to Item_field:

uint32 Item_type_holder::display_length(Item *item)

{

  if (item->type() == Item::FIELD_ITEM)

    return ((Item_field *)item)->max_disp_length();

  return ...; // the implementation for non-field Items

}

we'll remove the method Item_type_holder::display_length() and introduce a new virtual method in Item:

 virtual uint32 max_display_length() const

 {

   return type_handler()->max_display_length(this);

 }

Notice the default implementation will use max_display_length() of the underlying data type handler.
Item_field will override this implementation and read the result from the underlying field:

uint32 max_display_length() const { return field->max_display_length(); }