Split Item_func_hybrid_field_type::val_xxx() into methods in Type_handler

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

class Type_handler

{

  ...

  virtual

  String *Item_func_hybrid_field_type_val_str(Item_func_hybrid_field_type *,

                                                String *) const= 0;};

}

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

class Item_func_hybrid_field_type: public Item_hybrid_func

{

  ...

  String *val_str(String*str)

  {

    return Item_func_hybrid_field_type::type_handler()->

               Item_func_hybrid_field_type_val_str(this, str);

  }

  ...

}

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