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.
Sgtm.

Re `SecurityException`: Note that if we don't pick a ttl (= timer period) then we're stuck with caching forever, which is the opposite of your `"dns caching turned off"` message above—it would need to instead say something like `"dns cache refresh turned off"`.