private boolean mayScheduleCreateVisitorsTask() {
        return ! (scheduledSendCreateVisitors
                  || !progress.getIterator().hasNext()
                  || state.failed()
                  || enoughHitsReceived());
    }
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