Make CONTEXTUAL_DATA_KEY constant

It shouldn't be allowed to change it

Author: tsegismont

api/src/main/java/io/reactiverse/contextual/logging/impl/ContextualDataStorage.java

@@ -28,7 +28,7 @@
 public class ContextualDataStorage implements VertxServiceProvider {
 
   @SuppressWarnings("rawtypes")
-  static ContextLocal<ConcurrentMap> CONTEXTUAL_DATA_KEY = ContextLocal.registerLocal(ConcurrentMap.class);
+  final static ContextLocal<ConcurrentMap> CONTEXTUAL_DATA_KEY = ContextLocal.registerLocal(ConcurrentMap.class);
 
   @Override
   public void init(VertxBootstrap builder) {