!act,rem,clu #3549 Simplify and enhance routers

* Separate routing logic, to be usable stand alone, e.g. in actors
* Simplify RouterConfig, only a factory
* Move reading of config from Deployer to the RouterConfig
* Distiction between Pool and Group router types
* Remove usage of actorFor, use ActorSelection
* Management messages to add and remove routees
* Simplify the internals of RoutedActorCell & co
* Move resize specific code to separate RoutedActorCell subclass
* Change resizer api to only return capacity change
* Resizer only allowed together with Pool
* Re-implement all routers, and keep old api during deprecation phase
* Replace ClusterRouterConfig, deprecation
* Rewrite documentation
* Migration guide
* Also includes related ticket:
  +act #3087 Create nicer Props factories for RouterConfig

akka-actor/src/main/scala/akka/japi/JavaAPI.scala

@@ -210,4 +210,10 @@ object Util {
     }
 
   def immutableSingletonSeq[T](value: T): immutable.Seq[T] = value :: Nil
+
+  /**
+   * Turns an [[java.lang.Iterable]] into an immutable Scala IndexedSeq (by copying it).
+   */
+  def immutableIndexedSeq[T](iterable: java.lang.Iterable[T]): immutable.IndexedSeq[T] =
+    immutableSeq(iterable).toVector
 }