Fix exit being slowed by disk util stat time

Add a cv/lock to handle exit of the disk util thread. This is a
lot cleaner than the adhoc method that was used before, and it
means that we can signal the exit immediately instead of waiting
for DISKUTIL_MSEC to pass.

Signed-off-by: Jens Axboe <axboe@fb.com>
Get rid of (now unused) disk_util_mutex

Signed-off-by: Jens Axboe <axboe@fb.com>
backend: remove debug du thread exit notice

Signed-off-by: Jens Axboe <axboe@fb.com>
Start and stop status interval (or file) thread separately

This gets rid of the need to create threads on the fly for this,
and we can manage it and stop it before we do real stats at
the end.

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