Fix exit being slowed by disk util stat time

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>