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.
Oh, whoops, I thought the default in java was to cache forever.

My impression is that the way the cache works is that it checks whether it was last checked within `networkaddress.cache.ttl`, and if it was, it won't update.  DefaultTimer.twitter is a netty hashedwheeltimer with granularity of 10ms, so it's expected lag is 5ms, so my guess is that we'll have a lag of 5ms, but will otherwise get dns updates as fast as you specify in your security.

How about:

```scala
Try(Option(Security.getProperty("networkaddress.cache.ttl"))) match {
  case Throw(exc: SecurityException) => 
    log.error(Level.WARNING, "security permissions block us from checking ttl, dns caching turned off, change networkaddress.cache.ttl property permissions to enable proper finagle caching", exc)
    -1
  case Return(None) => // not sure if this is possible
  case Return(Some(value)) => value
}
```

If I'm reading InetAddress correctly, it sounds like there will always be a value set for `networkaddress.cache.ttl`.

I think it's easier to reason about by just using `networkaddress.cache.ttl`.  We can't get dns information faster than `networkaddress.cache.ttl` so it would necessarily be a slower DNS lookup--this seems potentially useful for people who want to turn off caching by setting it as 0, but not useful otherwise.