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 looked into what [java] does for this case, and it looks like it uses a `PrivilegedAction` to check, and doesn't try/catch on `SecurityException` (either it's impossible in a Privileged action or they're content to crash the process).  Maybe we should follow suit?  We could also pick a ttl, but it seems tricky to do that in a smart way when we don't know what java is going to be using.

I'm OK with not protecting against the pathological case.  That's why `Var.sampled` should not be used on a hot path when getting a value from `Var`.  It should be fine as long as we include a comment.

[0]: http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/sun/net/InetAddressCachePolicy.java#91