Misc changes, fixes and improvements after review.

- Renamed all 'frequency' to 'interval'
- Split up NodeJoinAndUpSpec and into NodeJoinSpec and NodeUpSpec.
- Split up MembershipChangeListenerJoinAndUpSpec and into MembershipChangeListenerJoinSpec and MembershipChangeListenerUpSpec.
- Added utility method 'startClusterNode()'
- Fixed race in register listener and telling node to leave
- Removed 'after' blocks
- Cleaned up unused code
- Improved comments

Signed-off-by: Jonas Bonér <jonas@jonasboner.com>

akka-cluster/src/multi-jvm/scala/akka/cluster/MultiNodeClusterSpec.scala

@@ -15,11 +15,11 @@ import akka.util.Duration
 object MultiNodeClusterSpec {
   def clusterConfig: Config = ConfigFactory.parseString("""
     akka.cluster {
-      auto-down                          = off
-      gossip-frequency                   = 200 ms
-      leader-actions-frequency           = 200 ms
-      unreachable-nodes-reaper-frequency = 200 ms
-      periodic-tasks-initial-delay       = 300 ms
+      auto-down                         = off
+      gossip-interval                   = 200 ms
+      leader-actions-interval           = 200 ms
+      unreachable-nodes-reaper-interval = 200 ms
+      periodic-tasks-initial-delay      = 300 ms
     }
     akka.test {
       single-expect-default = 5 s
@@ -29,8 +29,16 @@ object MultiNodeClusterSpec {
 
 trait MultiNodeClusterSpec { self: MultiNodeSpec ⇒
 
+  /**
+   * Create a cluster node using 'Cluster(system)'.
+   */
   def cluster: Cluster = Cluster(system)
 
+  /**
+   * Use this method instead of 'cluster.self'.
+   */
+  def startClusterNode(): Unit = cluster.self
+
   /**
    * Assert that the member addresses match the expected addresses in the
    * sort order used by the cluster.