blktrace: various bug fixes and improvements
- Use the normal debug logging for notify events, as they are expected
to happen.
- Correct a bug where a workload is considered mixed, even if it is
only reads (note: just cosmetical, no writes were issued by fio).
- Handle trim more appropriately - fio supports it as a separate
'data direction', so handle it as such.
- Fix a bug in the delay accounting, causing blktrace to not replay
correctly. This could cause either too fast replays, or replays
that would delay way too much.
- Consider TRIMs as writes, as far as --readonly being set is
concerned.
- Don't consider < 100 usec delays noise, fio can do this quite
accurately.
Signed-off-by: Jens Axboe <axboe@fb.com>