Adds local variants of pause and resume.

Created by: bradvogel

Enhances Queue#pause and Queue#resume methods to pause local workers.

This is useful for graceful shutdown. For example:

process.on('SIGUSR2', function() {
  // Stop processing new jobs.
  queue.pause(true /* Local */);

  // Wait for all currently processing jobs to complete (not covered in this snippet).
  onAllJobsComplete(function(){
    process.exit(1);
  });
});

Let me know if this works! Happy to make changes. I think it's incredibly useful as a step towards supporting graceful shutdown (a la https://github.com/Automattic/kue#graceful-shutdown).

Note that the only part of the API that I'm not completely happy with is that pause(true) only actually pauses after the current promise-loop has expired. So it's possible for another job to get processed after pause(true) has been called. This is usually fine for the graceful shutdown use case above. But perhaps you have a better idea.