Fix possible deadlock

- Removing some sync blocks which imho
  are not needed anymore.
- Adding some smoke tests.
- Relates to #364

spring-statemachine-samples/persist/src/test/java/demo/persist/PersistTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 the original author or authors.
+ * Copyright 2015-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -89,6 +89,25 @@ public class PersistTests {
 		assertThat(persist.listDbEntries(), containsString("id=2, state=SENT"));
 	}
 
+	@Test
+	public void testThreadingSmoke() throws Exception {
+		int cnt = 10;
+		Thread[] threads = new Thread[cnt];
+		for (int i = 0; i < cnt; i++) {
+			threads[i] = new Thread(() -> {
+				for (int j = 0; j < 1000; j++) {
+					persist.change(3, "SEND");
+				}
+			});
+		}
+		for (Thread t : threads) {
+			t.start();
+		}
+		for (Thread t : threads) {
+			t.join();
+		}
+	}
+
 	private static class TestListener extends StateMachineListenerAdapter<String, String> {
 
 		volatile CountDownLatch stateChangedLatch = new CountDownLatch(1);