成员的完整列表，这些成员属于 x1::xlt_position_info_t,包括所有继承而来的类成员

account_index	x1::xlt_position_info_t
exchange_index	x1::xlt_position_info_t
frozen_quantity	x1::xlt_position_info_t
security_code	x1::xlt_position_info_t
sellable_quantity	x1::xlt_position_info_t
total_quantity	x1::xlt_position_info_t