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.
How about something finite and small instead of `Duration.Top` for the default cache ttl? I think caching forever is a surprising default behavior in the setting of web services where dns names are typically used to (a) hide IPs that change over time and (b) load balance across multiple IPs at any fixed point in time. For example, a search for ["java dns cache"](https://www.google.com/search?q=java+dns+cache) primarily turns up instances of people asking "how do I turn off java dns caching?".

Maybe we could do something more inline with the dns caching behavior already present in `java.net.InetAddress`? I think that would provide the least surprising overall behavior.
- http://docs.oracle.com/javase/6/docs/api/java/net/InetAddress.html &rarr; "InetAddress Caching"
 - If security manager then cache successful lookups forever
 - If no security manager then cache successful lookups for a finite period
 - Regardless of security manager, always cache failed lookups for a "very short" finite period
 - Override above default behaviors with `networkaddress.cache.ttl` and `networkaddress.cache.negative.ttl`