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.
I think that in this case, it's a lot clearer to use standard constructs.

```
val d = try {
  val ttl = java.security.AccessController.doPrivileged(
    new PrivilegedAction[String]() {
      def run(): String = Security.getProperty("networkaddress.cache.ttl")
    }
  )
  ttl.toInt.seconds
} catch {
  case NonFatal(exc) => Duration.Top
}

if (!d.isFinite)
  log...
```