=clt #16948 Use min retries for singleton leaving scenario"

* In 2.4 we derive the number of hand-over/take-over retries from
  the removal margin, but we decided to set that to 0 by default, since
  it is intended for network partition scenarios. maxTakeOverRetries
  became 1. So there must be also be a  min number of retries property.
* The test failed for the leaving scenario because the singleton
  instance was stopped hard without sending the terminationMessage when
  the maxTakeOverRetries was exceeded.

akka-cluster-tools/src/multi-jvm/scala/akka/cluster/singleton/ClusterSingletonManagerSpec.scala

@@ -115,15 +115,21 @@ object ClusterSingletonManagerSpec extends MultiNodeConfig {
   /**
    * The Singleton actor
    */
-  class Consumer(queue: ActorRef, delegateTo: ActorRef) extends Actor {
+  class Consumer(queue: ActorRef, delegateTo: ActorRef) extends Actor with ActorLogging {
 
     import Consumer._
     import PointToPointChannel._
 
     var current = 0
+    var stoppedBeforeUnregistration = true
 
     override def preStart(): Unit = queue ! RegisterConsumer
 
+    override def postStop(): Unit = {
+      if (stoppedBeforeUnregistration)
+        log.warning("Stopped before unregistration")
+    }
+
     def receive = {
       case n: Int if n <= current ⇒
         context.stop(self)
@@ -138,6 +144,7 @@ object ClusterSingletonManagerSpec extends MultiNodeConfig {
       case End ⇒
         queue ! UnregisterConsumer
       case UnregistrationOk ⇒
+        stoppedBeforeUnregistration = false
         context stop self
       case Ping ⇒
         sender() ! Pong