code: func (s *StanServer) setupAckTimer(sub *subState, d time.Duration) {
  	sub.ackTimer = time.AfterFunc(d, func() {
  		s.performAckExpirationRedelivery(sub, false)
  	})
  }
docs: // Sets up the ackTimer to fire at the given duration.
  // sub's lock held on entry.