Add exit_on_io_done option to the CPU IO engine

Add exit_on_io_done option to the CPU IO engine

The CPU IO engine is most often used to saturate the system,
while running an IO load on it. As such, it's useful to have
CPU engine threads exit automatically, when IO has completed.
Add exit_on_io_done as a CPU IO engine option for that purpose.

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