=clu replace Set -- with diff and ++ with union

* better performance according to
  https://docs.google.com/presentation/d/1Qjryxoe-fYEM8ZPhM-98LKfbhnRcn5eAEMNlVVnixsA/pub

akka-cluster-metrics/src/main/scala/akka/cluster/metrics/ClusterMetricsCollector.scala

@@ -211,7 +211,7 @@ private[metrics] class ClusterMetricsCollector extends Actor with ActorLogging {
    * Updates the initial node ring for those nodes that are [[akka.cluster.MemberStatus]] `Up`.
    */
   def receiveState(state: CurrentClusterState): Unit =
-    nodes = (state.members -- state.unreachable) collect {
+    nodes = (state.members diff state.unreachable) collect {
       case m if m.status == MemberStatus.Up || m.status == MemberStatus.WeaklyUp ⇒ m.address
     }