Initialize the default event bus statically

The previous method that used double checked synchronization offers
little performance benefits over a plain static initialization.

Reported-by @harkle-the-cake

event-bus-core/src/main/java/dev/kske/eventbus/core/EventBus.java

@@ -52,7 +52,7 @@ public final class EventBus {
 	 */
 	public static final int DEFAULT_PRIORITY = 100;
 
-	private static EventBus singletonInstance = new EventBus();
+	private static final EventBus singletonInstance = new EventBus();
 
 	private static final Logger logger = System.getLogger(EventBus.class.getName());