@Override
    public boolean add(E o) {
        return map.putIfAbsent(o, Boolean.TRUE) == null;
    }
add an obj to set, if exist, return false, else return true
@see java.util.AbstractCollection#add(Object)
jraft-core/src/main/java/com/alipay/sofa/jraft/util/concurrent/ConcurrentHashSet.java