stats: Add a function to report completion latency percentiles

stats: Add a function to report completion latency percentiles

    This patch introduces two new options:
    1) Specifying --clat_percentiles enables the reporting of
       completion latency percentiles.
    2) --percentile_list allows the user to customize the
       list of percentiles to report.

    Signed-off-by: Yu-ju Hong <yjhong@google.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>