Split Item_func_min_max::val_xxx() and Item_func_min_max::get_date() into methods in Type_handler

Under term of this tasks we'll remove tests for cmp_type() and result_type() in Item_func_min_max::val_xxx() and Item_func_min_max::get_date() and introduce instead new methods in Type_handler, e.g.:

class Type_handler

{

  ...

  virtual

  String *Item_func_min_max_val_str(Item_func_min_max *,

                                    String *) const;

}

The methods Item_func_hybrid_min_max::val_xxx() and Item_func_min_max::get_date() will call the corresponding new methods in Type_handler:

class Item_func_min_max: public Item_hybrid_func

{

  ...

  String *val_str(String*str)

  {

    return Item_func_min_max::type_handler()->

             Item_func_min_max_val_str(this, str);

  }

  ...

}

For performance purposes, the change should try not to introduce new virtual calls.
One virtual call for xxx_type() should be replaced to one virtual call in Type_handler.
All possible calls inside the new methods should be done statically.