get_next

获取扫描得到的数据，需要循环执行，直到返回None结束扫描。

  1. def get_next(self):
  2.     """
  3.     scan the next k-v pair for the scanner.
  5.     :return: (tuple<tuple<hash_key, sort_key>, value> or None)
  6.              all the sort_keys returned by this API are in ascend order.
  7.     """