Ability to exit work loop gracefully

[rgarcia]

In order to gracefully remove a worker, there needs to be a way to stop asking for more work. Other language libraries have this functionality, e.g.

• PHP has a $monitor callback: https://github.com/brianlmoon/net_gearman/blob/master/Net/Gearman/Worker.php. The
• Perl has a stop_if callback: https://github.com/gearman/perl-Gearman-Client/blob/master/t/worker.pl