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.
try something like:

    Var.async[Addr](Addr.Bound(parseHosts)) { u =>
      timer.schedule(...) { 
        futurePool(parseHosts) onSuccess { addrs =>
          u() = Addr.Bound(addrs)
        } onFailure { ... }
      }
    }