@Override
    public void ack(AckToken token) {
        if (log.isLoggable(Level.FINE)) {
            log.log(Level.FINE, "Visitor session " + sessionName +
                    ": Sending ack " + token.ackObject);
        }
        synchronized (stateMonitor) {
            decrementLocallyProcessingCounter();
        }
        // No locking here; replying should be thread safe in itself
        receiver.reply((Reply)token.ackObject);
    }
Schedule a new SendCreateVisitors task iff there are still buckets to
visit, the visiting has not failed fatally and we haven't already
scheduled such a task. Return whether a visitor was scheduled here.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSession.java