Cleanup logging

If the log knows the filename it is supposed to log to, then we
don't need to have part of that on the init side and the other
part on the flush side. So clean it up so that the caller of
setup_log() provides everything we need. This gets rid of
various ways of ending the log, and opens us up to doing
log flushing mid-run.

Signed-off-by: Jens Axboe <axboe@fb.com>
--latency-log is now gone

Use per-job options for this.

Signed-off-by: Jens Axboe <axboe@fb.com>
cconv: convert ->log_offset on the wire

Forgotten in commit ae58885234

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