#3370 - Undeprecating Props.apply(=> Actor) and Props.withCreator(=> Actor)

Also removing the lazy val implementation of Props.constructor and Props.cachedActorClass
Merge pull request #1454 from akka/wip-3370-undeprecate-creator-√

 #3370 - Undeprecating Props.apply(=> Actor) and Props.withCreator(=> Ac...