Try to be smart about the handler

Created by: albertjan

This tries to resolve the easy to make mistake with specifying a handler with a callback and returning a promise.

It's a bit intricate but it does the job 😄.

This will make these work:

queue.process(function(job, done) {
  return Promise.resolve();
});

and

queue.process(function(job, done) {
  return 5;
});

This one will probably create a race condition (for as far as possible in js):

queue.process(function(job, done) {
   done(5);
   return Promise.resolve(6);
});

And this one will outright not work for obvious reasons:

queue.process(function(job, done) {
  setTimeout(done, 500);
  return 4;
});