code: @Override
  	public void purge(long tillWhen){
  		Long t;
  		while((t = ((NavigableMap<Long, AtomicLong>)counters.getMap()).firstKey()) != null && t < tillWhen){
  			counters.remove(t);
  		}
  	}
docs: 删除掉早于一定时间的记录
  @param tillWhen	清除到哪个时间点为止