clear() {
    this.jobs
      .filter(job => job.status === Job.status.READY)
      .forEach(job => {
        job.failureListeners.forEach(failureListener => {
          failureListener({
            code: 'QUEUE_CLEARED',
            message: 'This job was terminated since the queue was cleared',
            job,
          })
        })
      })
    this.jobs = []
  }
Clear the queue of all "ready" jobs. Jobs
in execution are not terminate prematurely.
Jobs being terminated have their failure listeners notified.