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.
we already have most of this behavior in `[InetSocketAddressUtill][0]`.  Maybe just change InetSocketAddressUtil to use getAllByName in parseHosts?

[0]: https://github.com/twitter/finagle/blob/master/finagle-core/src/main/scala/com/twitter/finagle/util/InetSocketAddressUtil.scala#L29