docs: public Value putIfAbsent(T key, long expiry_time) {
          if(key == null)
              key=NULL_KEY;
          Value val=map.get(key);
          if(val == null) {
              val=new Value();
              Value existing=map.putIfAbsent(key, val);
              if(existing == null)
                  return val;
              val=existing;
          }
          // key already exists
          if(val.update().age() > expiry_time) {
              map.remove(key);
              map.putIfAbsent(key, new Value());
              return val;
          }
          return null;
      }