=rem #3867 Make sure endpointWriter is terminated before ungate

* actor name [endpointWriter] is not unique
* The problem was that the test used 100ms until ungate and
  that made it possible for the endpointWriter to not be
  completely terminated (and removed) before Ungate and
  new Send in idle state, which created new endpointWriter
* I could reproduce it with a sleep in EndpointWriter.postStop
* The solution is to start the scheduled Ungate after Terminated
  has been received

akka-remote/src/main/scala/akka/remote/Endpoint.scala

@@ -211,8 +211,6 @@ private[remote] class ReliableDeliverySupervisor(
         remoteAddress, settings.RetryGateClosedFor.toMillis, e.getMessage)
       uidConfirmed = false // Need confirmation of UID again
       context.become(gated)
-      context.system.scheduler.scheduleOnce(settings.RetryGateClosedFor, self, Ungate)
-      context.unwatch(writer)
       currentHandle = None
       context.parent ! StoppedReading(self)
       Stop
@@ -319,6 +317,8 @@ private[remote] class ReliableDeliverySupervisor(
   }
 
   def gated: Receive = {
+    case Terminated(_) ⇒
+      context.system.scheduler.scheduleOnce(settings.RetryGateClosedFor, self, Ungate)
     case Ungate ⇒
       if (resendBuffer.nonAcked.nonEmpty || resendBuffer.nacked.nonEmpty) {
         writer = createWriter()