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>