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.
Since this is the way that the jdk learns about its cache ttl, I don't know that it would make sense to have an upper bound on the ttl (since it can't possibly have changed before the upper bound).  A lower bound would probably be reasonable--a TTL of 0 won't work well with our current system.