public boolean startBarrier(BarrierWaiter handler) {
        if (eventCount == 0 && queue.isEmpty()) {
            return false;
        }
        queue.add(new Entry(eventCount, handler));
        ++barrierToken;
        eventCount = 0;
        return true;
    }
Initiate the detection of the minimal event barrier starting
now. If this method returns false it means that no events were
currently active and the minimal event barrier was infinitely
small. If this method returns false the handler will not be
notified of the completion of the barrier. If this method
returns true it means that the started barrier is pending and
that the handler passed to this method will be notified of its
completion at a later time.

@param handler Handler notified of the completion of the barrier.
@return True if a barrier was started, false if no events were active.
vespajlib/src/main/java/com/yahoo/concurrent/EventBarrier.java