async_del
异步删除单条数据
///
/// \brief asynchronous del
/// del stored k-v by key from cluster
/// key is composed of hashkey and sortkey. must provide both to get the value.
/// will not be blocked, return immediately.
/// \param hashkey
/// used to decide from which partition to del this k-v
/// \param sortkey
/// all the k-v under hashkey will be sorted by sortkey.
/// \param callback
/// the callback function will be invoked after operation finished or error occurred.
/// \param timeout_milliseconds
/// if wait longer than this value, will return time out error
/// \return
/// void.
///
virtual void async_del(const std::string &hashkey,
const std::string &sortkey,
async_del_callback_t &&callback = nullptr,
int timeout_milliseconds = 5000) = 0;