Support joining 2.5.9 or earlier, compat InitJoinAck, #25491

* Detect that joining node is 2.5.9 or earlier by empty ConfigCheck
  config in InitJoin message. Then send back Address, which was the
  old representation of InitJoinAck
* Include akka.version in logging to facilitate troubleshooting

akka-cluster/src/main/scala/akka/cluster/Cluster.scala

@@ -99,7 +99,7 @@ class Cluster(val system: ExtendedActorSystem) extends Extension {
   // ClusterJmx is initialized as the last thing in the constructor
   private var clusterJmx: Option[ClusterJmx] = None
 
-  logInfo("Starting up...")
+  logInfo("Starting up, Akka version [{}] ...", system.settings.ConfigVersion)
 
   val failureDetector: FailureDetectorRegistry[Address] = {
     val createFailureDetector = () ⇒