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/NodeMembershipSpec.scala

@@ -22,7 +22,11 @@ class NodeMembershipMultiJvmNode1 extends NodeMembershipSpec
 class NodeMembershipMultiJvmNode2 extends NodeMembershipSpec
 class NodeMembershipMultiJvmNode3 extends NodeMembershipSpec
 
-abstract class NodeMembershipSpec extends MultiNodeSpec(NodeMembershipMultiJvmSpec) with MultiNodeClusterSpec with ImplicitSender with BeforeAndAfter {
+abstract class NodeMembershipSpec
+  extends MultiNodeSpec(NodeMembershipMultiJvmSpec)
+  with MultiNodeClusterSpec
+  with ImplicitSender with BeforeAndAfter {
+
   import NodeMembershipMultiJvmSpec._
 
   override def initialParticipants = 3
@@ -41,7 +45,7 @@ abstract class NodeMembershipSpec extends MultiNodeSpec(NodeMembershipMultiJvmSp
 
       // make sure that the node-to-join is started before other join
       runOn(first) {
-        cluster.self
+        startClusterNode()
       }
       testConductor.enter("first-started")