get_scanner
获取针对某个hashkey下的sortkey区间 [sortkeyA ~ sortkeyB)的一个scanner
///
/// \brief get hash scanner
/// get scanner for [start_sortkey, stop_sortkey) of hashkey
/// \param hashkey
/// cannot be empty
/// \param start_sortkey
/// sortkey to start with
/// \param stop_sortkey
/// sortkey to stop. ""(empty string) represents the max key
/// \param options
/// which used to indicate scan options, like which bound is inclusive
/// \param scanner
/// out param, used to get k-v
/// this pointer should be deleted when scan complete
/// \return
/// int, the error indicates whether or not the operation is succeeded.
/// this error can be converted to a string using get_error_string()
///
virtual int get_scanner(const std::string &hashkey,
const std::string &start_sortkey, // start from beginning if this set ""
const std::string &stop_sortkey, // to the last item if this set ""
const scan_options &options,
pegasus_scanner *&scanner) = 0;