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/ClientDowningNodeThatIsUnreachableSpec.scala

@@ -26,8 +26,8 @@ class ClientDowningNodeThatIsUnreachableMultiJvmNode4 extends ClientDowningNodeT
 
 class ClientDowningNodeThatIsUnreachableSpec
   extends MultiNodeSpec(ClientDowningNodeThatIsUnreachableMultiJvmSpec)
-  with MultiNodeClusterSpec
-  with ImplicitSender with BeforeAndAfter {
+  with MultiNodeClusterSpec {
+
   import ClientDowningNodeThatIsUnreachableMultiJvmSpec._
 
   override def initialParticipants = 4
@@ -36,7 +36,7 @@ class ClientDowningNodeThatIsUnreachableSpec
 
     "be able to DOWN a node that is UNREACHABLE (killed)" taggedAs LongRunningTest in {
       runOn(first) {
-        cluster.self
+        startClusterNode()
         awaitUpConvergence(numberOfMembers = 4)
 
         val thirdAddress = node(third).address