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.
From my testing, it seems the "assert" is executed and means there is a change.
Looking at the code here: https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Var.scala#L214,
the value will always be published each time the event is observed.
If I understand something wrong, please point out.