engines/net: add subjob number to given port

If we use numjobs=x where x > 1, then we don't want to reuse the
same port. Bump it within a group, using the subjob number.

Signed-off-by: Jens Axboe <axboe@fb.com>
Update documentation on net engine port usage

Signed-off-by: Jens Axboe <axboe@fb.com>
stat: add total/short/drop ios to the json output

Signed-off-by: Jens Axboe <axboe@fb.com>
engines/libaio: better protect against a busy loop in getevents()

If we reap nothing and we were asked to reap > 0 events, then
reuse our commit, sleep, and retry loop.

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