finagle-core: make timer-based DNS resolution as default of InetResolver

I have tested on my macbook and the timer-based DNS resolution works fine.
Ok, I got your point. So what is the proper way to get notified when observation is closed?
I looked at the code here:https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Var.scala#L414-L418,
but I didn't get the point how I can register timerTask.close().