Simple JDK 11 fixes (#26038)

* TLSSpec fix for Java 11 #25739

* =act silence logging in AsyncDns specs

* +act Add internal akka.util.JavaVersion for determining runtime Java version

* =act #25733 run TcpIntegrationSpec peers on different ActorSystems

* pro: add explicit dependency to activation when using dockerClient for JDK 11+

Otherwise, the log is spammed with lots of ClassNotFound exceptions when
running AsyncDnsResolverIntegrationSpec

akka-actor/src/main/scala/akka/util/JavaVersion.scala

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2009-2018 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.util
+
+import akka.annotation.InternalApi
+
+/**
+ * INTERNAL API
+ */
+@InternalApi private[akka] object JavaVersion {
+
+  val majorVersion: Int = {
+    // FIXME replace with Runtime.version() when we no longer support Java 8
+    // See Oracle section 1.5.3 at:
+    // https://docs.oracle.com/javase/8/docs/technotes/guides/versioning/spec/versioning2.html
+    val version = System.getProperty("java.specification.version").split('.')
+
+    val majorString =
+      if (version(0) == "1") version(1) // Java 8 will be 1.8
+      else version(0) // later will be 9, 10, 11 etc
+
+    majorString.toInt
+  }
+}