Add on_loop_stop call in finally block for a pool process worker's main loop

This change is necessary for an upcoming pull request into celery that will fire off an on_worker_process_shutdown signal when a pool worker exits. I look forward to comments or concerns about this suggested change in billiard so I can ensure my current and future submissions comply with project guidelines. 

I do not need the exitcode for said change to celery, but I thought it would be worthwhile to make a more complete feature possible.