* feat: add search key debug command * feat: modify to merge default cf and write cf keys * feat: modify to read backup meta files concurrently * feat: add unit test and comments * fix: check error returned from task.GetStorage * feat: resolve reviewing problems * fix: modify removed function Co-authored-by: Zak Zhao <57036248+joccau@users.noreply.github.com> Co-authored-by: Weizhen Wang <wangweizhen@pingcap.com> Co-authored-by: 3pointer <luancheng@pingcap.com>