def push(j)
      raise "invalid work" if j == nil
      j.job_id = IDManager.get if j.job_id == nil
      j.group = self
      @queue.push(j)
      @jobs_by_id[j.job_id] = j
    end
def push(j)
      raise "invalid work" if j == nil
      j.job_id = IDManager.get if j.job_id == nil
      j.group = self
      @queue.push(j)
      @jobs_by_id[j.job_id] = j
    end
Add something to the work queue