Error: Shutdown already in progress - critical issues with Kue

Seems calling `queue.shutdown() `when more that one queue exists causes an error:

`Error: Shutdown already in progress`

See example below:

```
process.stdin.resume();

const kue = require('kue');

let queue = kue.createQueue({
redis: 'redis://localhost:6380'
});

queue.process('my-event', (job, done) => {
console.info(job.data);

done();
});

//_________________________________________________________________________________________________

let queue2 = kue.createQueue({
redis: 'redis://localhost:6380'
});

queue2.process('my-other-event', (job, done) => {
console.info(job.data);

done();
});

//_________________________________________________________________________________________________

let job = queue
.create('my-event', {foo: 'bar'})
.removeOnComplete(true)
.save();

setTimeout(() => {
queue.shutdown(0, (err) => {
if (err) {
return console.error(err);
}

console.info('queue shutdown');
});

}, 5000);

let job2 = queue2
.create('my-other-event', {foo: 'bar'})
.removeOnComplete(true)
.save();

setTimeout(() => {
queue2.shutdown(0, (err) => {
if (err) {
return console.error(err);
}

console.info('queue2 shutdown');
});

}, 5000);

```
While I am grateful for OS contribution, there is so much critical stuff that in undocumented in this project

The issue provides no file or test names. Start by running the supplied two-queue Node.js/Redis reproduction and trace queue.shutdown(0) for both queue instances; done means each callback completes without “Shutdown already in progress” while both workers stop cleanly.

Written by the indexing model from the issue text.