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.
On second thought, the second part of my comment above isn't a good idea: the jvm is already caching dns resolutions for us, so we shouldn't try to "replicate" any of it.

One odd behavior above: if we reuse `networkaddress.cache.ttl`, then we effectively increase the user-requested cache ttl by 1-2x, since we're composing two independent timed processes. Is that desired? If not, would it be good enough to set a fixed, small `ttl` like `10.seconds` and leave it at that? Or maybe expose a new config to override it instead of piggybacking on `networkaddress.cache.ttl`?