+act #15502: Pluggable DNS resolution in akka-io

This is an initial step towards fully asynchronous DNS resolution.
Right now it improves things by loading the resolution off to a separate actor with potentially dedicated dispatcher.
The code is covered by TcpConnectionSpec "report failed connection attempt when target cannot be resolved", although only unsuccessful resolution is tested.

Fully asynchronous DNS client coming soon, it's already working in production, but isn't 100% RFC compliant.