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/JoinTwoClustersSpec.scala

@@ -27,7 +27,10 @@ class JoinTwoClustersMultiJvmNode4 extends JoinTwoClustersSpec
 class JoinTwoClustersMultiJvmNode5 extends JoinTwoClustersSpec
 class JoinTwoClustersMultiJvmNode6 extends JoinTwoClustersSpec
 
-abstract class JoinTwoClustersSpec extends MultiNodeSpec(JoinTwoClustersMultiJvmSpec) with MultiNodeClusterSpec with ImplicitSender {
+abstract class JoinTwoClustersSpec
+  extends MultiNodeSpec(JoinTwoClustersMultiJvmSpec)
+  with MultiNodeClusterSpec {
+
   import JoinTwoClustersMultiJvmSpec._
 
   override def initialParticipants = 6
@@ -41,7 +44,7 @@ abstract class JoinTwoClustersSpec extends MultiNodeSpec(JoinTwoClustersMultiJvm
     "be able to 'elect' a single leader after joining (A -> B)" taggedAs LongRunningTest in {
       // make sure that the node-to-join is started before other join
       runOn(a1, b1, c1) {
-        cluster.self
+        startClusterNode()
       }
       testConductor.enter("first-started")