=clu #3600 Disable Cluster assertInvariants checks by default

* asserts can be enabled by system property 'akka.cluster.assert=on'

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

@@ -39,6 +39,15 @@ object Cluster extends ExtensionId[Cluster] with ExtensionIdProvider {
   override def lookup = Cluster
 
   override def createExtension(system: ExtendedActorSystem): Cluster = new Cluster(system)
+
+  /**
+   * INTERNAL API
+   */
+  private[cluster] final val isAssertInvariantsEnabled: Boolean =
+    System.getProperty("akka.cluster.assert", "off").toLowerCase match {
+      case "on" | "true" ⇒ true
+      case _             ⇒ false
+    }
 }
 
 /**

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

@@ -255,8 +255,7 @@ private[cluster] case class ClusterHeartbeatSenderState private (
   ending: Set[Address] = Set.empty,
   heartbeatRequest: Map[Address, Deadline] = Map.empty) {
 
-  // TODO can be disabled as optimization
+  if (Cluster.isAssertInvariantsEnabled) assertInvariants()
-  assertInvariants()
 
   private def assertInvariants(): Unit = {
     val currentAndEnding = current.intersect(ending)

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

@@ -63,8 +63,7 @@ private[cluster] case class Gossip(
   overview: GossipOverview = GossipOverview(),
   version: VectorClock = VectorClock()) { // vector clock version
 
-  // TODO can be disabled as optimization
+  if (Cluster.isAssertInvariantsEnabled) assertInvariants()
-  assertInvariants()
 
   private def assertInvariants(): Unit = {
 

akka-cluster/src/test/scala/akka/cluster/GossipSpec.scala

@@ -60,15 +60,6 @@ class GossipSpec extends WordSpec with MustMatchers {
       merged2.overview.reachability.allUnreachable must be(merged1.overview.reachability.allUnreachable)
     }
 
-    "not have live members with wrong status" in intercept[IllegalArgumentException] {
-      // b2 is Removed
-      Gossip(members = SortedSet(a2, b2))
-    }
-
-    "not have non cluster members in seen table" in intercept[IllegalArgumentException] {
-      Gossip(members = SortedSet(a1, e1)).seen(a1.uniqueAddress).seen(e1.uniqueAddress).seen(b1.uniqueAddress)
-    }
-
     "have leader as first member based on ordering, except Exiting status" in {
       Gossip(members = SortedSet(c2, e2)).leader must be(Some(c2.uniqueAddress))
       Gossip(members = SortedSet(c3, e2)).leader must be(Some(e2.uniqueAddress))