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.
`AccessController.doPrivileged` may throw `AccessControlException` in some configurations. I suggest catching it here. Not catching it will render `InetResolver` unusable in those configurations. 

I'm not sure other parts of Finagle would work correctly in restricted environments, but let's not add +1 point of failure :)