Improve logging accuracy

Improve logging accuracy

If we do windowed logging (over a period of time), then we can
take the logging out of the IO worker fast path, and just have
our helper thread do it. This means we'll reduce the overhead
for the IO worker (which is important), and that we'll be able
to provide better accuracy of when we log. Before this change,
we'd log when we had exceeded the window of time at a completion
even. This means that if you had a logging time of 500 msec,
you'd always exceed that and even tend to skew it to being much
later. With this change, logging should be very accurate time
wise:

$ ./fio --inflate-log=fsrandW4-8-iopslog_iops.1.log.fz | head -n10
500, 470, 0, 0
1000, 492, 0, 0
1500, 484, 0, 0
2000, 492, 0, 0
2500, 454, 0, 0
3000, 462, 0, 0
3500, 480, 0, 0
4000, 462, 0, 0
4500, 472, 0, 0
5000, 496, 0, 0

and not exhibit any drift at all.

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