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.
Basically, the problem is that after I've scheduled this, it will sit around on the timer forever, even if I no longer care about these `InetSocketAddress`es.  Imagine that I have a web crawler which takes hostnames and crawls them.  Every time I do a DNS lookup of a hostname to begin crawling, I add a task to the timer to be executed again and again.  It would be better if the tasks could be removed from the timer when they are no longer used.