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.
From the doc: http://docs.oracle.com/javase/7/docs/api/java/security/AccessController.html#doPrivileged(java.security.PrivilegedAction)
<code>AccessController.doPrivileged</code> may only throws <code>NullPointerException</code>
@sirmax Do you have any experience that <code>AccessControlException</code> is thrown?