Disallow re-joining, see #2873

* Disallow join requests when already part of a cluster
* Remove wipe state when joining, since join can only be
  performed from empty state
* When trying to join, only accept gossip from that member
* Ignore gossips from unknown (and unreachable) members
* Make sure received gossip contains selfAddress
* Test join of fresh node with same host:port
* Remove JoinTwoClustersSpec
* Welcome message as reply to Join
* Retry unsucessful join request
* AddressUidExtension
* Uid in cluster Member identifier
  To be able to distinguish nodes with same host:port
  after restart.
* Ignore gossip with wrong uid
* Renamed Remove command to Shutdown
* Use uid in vclock identifier
* Update sample, Member apply is private
* Disabled config duration syntax and cleanup of io settings
* Update documentation

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

@@ -0,0 +1,29 @@
+/**
+ * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.remote
+
+import scala.concurrent.forkjoin.ThreadLocalRandom
+import akka.actor.ActorSystem
+import akka.actor.ExtendedActorSystem
+import akka.actor.Extension
+import akka.actor.ExtensionId
+import akka.actor.ExtensionIdProvider
+
+/**
+ * Extension that holds a uid that is assigned as a random `Int`.
+ * The uid is intended to be used together with an [[akka.actor.Address]]
+ * to be able to distinguish restarted actor system using the same host
+ * and port.
+ */
+object AddressUidExtension extends ExtensionId[AddressUidExtension] with ExtensionIdProvider {
+  override def get(system: ActorSystem): AddressUidExtension = super.get(system)
+
+  override def lookup = AddressUidExtension
+
+  override def createExtension(system: ExtendedActorSystem): AddressUidExtension = new AddressUidExtension(system)
+}
+
+class AddressUidExtension(val system: ExtendedActorSystem) extends Extension {
+  val addressUid: Int = ThreadLocalRandom.current.nextInt()
+}