Better handling of system versioning for generated stored columns

field_def:

          opt_attribute

        | opt*generated_always AS virtual_column*func

         {

           Lex->last*field->vcol*info= $3;

           Lex->last*field->flags&= ~NOT_NULL*FLAG; // undo automatic NOT NULL for timestamps

         }

          vcol*opt_specifier vcol_opt*attribute

        | opt*generated_always AS ROW_SYM START_SYM opt_asrow*attribute

          {

            if (Lex->last*field_generated_always_as_row*start())

              MYSQL_YYABORT;

          }

        | opt*generated_always AS ROW_SYM END opt_asrow*attribute

          {

            if (Lex->last*field_generated_always_as_row*end())

              MYSQL_YYABORT;

          }

        ;