stat: set kb/unit_base, if not already set

Signed-off-by: Jens Axboe <axboe@fb.com>
client: always output summed output for all clients

At least if we have more than 1 client.

Signed-off-by: Jens Axboe <axboe@fb.com>
client: get rid of (now) unused client->nr_stat

Signed-off-by: Jens Axboe <axboe@fb.com>