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.
This is a bit of an overkill. I think it'd be simpler & clearer with more boring code:

```
val ttl = Security.getProperty("networkaddress.cache.ttl")
if (t/l == null) defaultTtl
else t/l.toInt.seconds max minttl min maxTtl
```