return all market quotation snapshot
        :param prefix: if prefix is True, return quotation dict's  stock_code
             key start with sh/sz market flag

def market_snapshot(self, prefix=False):
        """return all market quotation snapshot
        :param prefix: if prefix is True, return quotation dict's  stock_code
             key start with sh/sz market flag
        """
        return self.get_stock_data(self.stock_list, prefix=prefix)