radosgw-admin: output bucket sync checkpoint to stdout

radosgw-admin: output bucket sync checkpoint to stdout

The bucket sync checkpoint command was outputting its results to
the log stream via ldpp_dout() instead of stdout. This meant users
saw no output unless --log-to-stderr was explicitly enabled.

Change success and completion messages to std::cout and errors
to std::cerr so results are visible without enabling logging.

Fixes: http://tracker.ceph.com/issues/72088
Signed-off-by: cheese-cakee <farzanaman99@gmail.com>