Fix rbd bench-write improper behavior

"rbd bench-write" eject all write operations with the same offset at the same
time. It will result in non-objective performance result from this command.

Make performance statistic more exact with header title displayed.

fix #7066

Co-Author: Rongze Zhu <rongze@unitedstack.com>
Signed-off-by: Haomai Wang <haomaiwang@gmail.com>