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.
Thanks for the review. I think I can change the code to align all the comments by tomorrow. 
But for this comment, I am not very sure about the problem(memory leak and timer task leak). Can you help to explain more on this? Any example code in existing code base will definitely help me.
Thanks.