+act #3529 Add convenience helper for looking up an actor by identity

* ActorSelection.resolveOne
(cherry picked from commit 37c4053b2973b1ade089cd9e02f5a6807f974838)
Merge pull request #1686 from akka/wip-3529-identify-sugar-2.2-patriknw

+act #3529 Add convenience helper for looking up an actor by identity (backport)