@Override
    public boolean isDone() {
        synchronized (stateMonitor) {
            return done;
        }
    }
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