Pause object has a static ExecutorService that can't be shutdown

`org.assertj.swing.timing.Pause` has an ExecutorService. It's static and cannot be closed. It means that we have non-daemon threads hanging in there and preventing the JVM to finish.

An example is
```java
public static void main(String[] args) throws Exception {
Pause.pause(new Condition("Cond") {
@Override
public boolean test() {
return true;
}
});

Field field = Pause.class.getDeclaredField("EXECUTOR_SERVICE");
field.setAccessible(true);
ExecutorService service = (ExecutorService) field.get(null);
service.shutdownNow();
}
```

Without the reflection code at the end, it won't exit for a while.

The easiest solution is to add a static `close()` method to shutdown the pool. But I think the best is to change the code to rely on the commonPool, e.g. `ForkJoinPool.commonPool().submit()` or a CompletableFuture.