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.
Personally no, I don't have such experience. But it seemed odd to me that one can safely elevate permissions, so I googled "AccessController.doPrivileged AccessControlException" and found a some cases when it does happen. One was a sandboxed applet, another had a security manager installed with restrictive permissions.

In the doc on `AccessController` itself there is a passage which hints that things may go wrong:

> If that caller's domain has the specified permission, no further checking is done and checkPermission returns quietly, indicating that the requested access is allowed. If that domain does not have the specified permission, an exception is thrown, as usual.