Performance issue when waiting for half sec for job to finish before continuing

This line was added to pause for a half a second to conserve system resources
https://github.com/resque/php-resque/blob/050a337532d918c1a0ef49a03db6a508601ea544/lib/Worker/ResqueWorker.php#L287

However that makes processing jobs very slow. Changing that to say 500 improves queue processing performance drastically.

Making this configurable would allow the user to change it.