Synchronize Join PseudoState state machine listener

- Synchronize JoinPseudoState's
  JoinTracker.stateChanged(State<S, E>, State<S, E>) method
  for playing nice with concurrent region execution.
  Should fix #93.
- Tuning test which relates to #76. What comes for the
  tests, there was this concurrency issue and also problem
  in tests itself.
- Polish JoinPseudoState.

spring-statemachine-samples/tasks/src/test/java/demo/tasks/TasksTests.java

@@ -156,6 +156,8 @@ public class TasksTests {
 
 	static class TestListener extends StateMachineListenerAdapter<States, Events> {
 
+		final Object lock = new Object();
+
 		volatile CountDownLatch stateChangedLatch = new CountDownLatch(1);
 		volatile CountDownLatch stateEnteredLatch = new CountDownLatch(2);
 		volatile CountDownLatch stateExitedLatch = new CountDownLatch(0);
@@ -167,26 +169,34 @@ public class TasksTests {
 
 		@Override
 		public void stateChanged(State<States, Events> from, State<States, Events> to) {
-			stateChangedCount++;
-			stateChangedLatch.countDown();
+			synchronized (lock) {
+				stateChangedCount++;
+				stateChangedLatch.countDown();
+			}
 		}
 
 		@Override
 		public void stateEntered(State<States, Events> state) {
-			statesEntered.add(state);
-			stateEnteredLatch.countDown();
+			synchronized (lock) {
+				statesEntered.add(state);
+				stateEnteredLatch.countDown();
+			}
 		}
 
 		@Override
 		public void stateExited(State<States, Events> state) {
-			statesExited.add(state);
-			stateExitedLatch.countDown();
+			synchronized (lock) {
+				statesExited.add(state);
+				stateExitedLatch.countDown();
+			}
 		}
 
 		@Override
 		public void transitionEnded(Transition<States, Events> transition) {
-			transitionCount++;
-			transitionLatch.countDown();
+			synchronized (lock) {
+				transitionCount++;
+				transitionLatch.countDown();
+			}
 		}
 
 		public void reset(int c1, int c2, int c3) {
@@ -194,14 +204,16 @@ public class TasksTests {
 		}
 
 		public void reset(int c1, int c2, int c3, int c4) {
-			stateChangedLatch = new CountDownLatch(c1);
-			stateEnteredLatch = new CountDownLatch(c2);
-			stateExitedLatch = new CountDownLatch(c3);
-			transitionLatch = new CountDownLatch(c4);
-			stateChangedCount = 0;
-			transitionCount = 0;
-			statesEntered.clear();
-			statesExited.clear();
+			synchronized (lock) {
+				stateChangedLatch = new CountDownLatch(c1);
+				stateEnteredLatch = new CountDownLatch(c2);
+				stateExitedLatch = new CountDownLatch(c3);
+				transitionLatch = new CountDownLatch(c4);
+				stateChangedCount = 0;
+				transitionCount = 0;
+				statesEntered.clear();
+				statesExited.clear();
+			}
 		}
 
 	}