clear the UserStoreImpl.listeners map in the UserStoreImpl.clear() method; fresh replicas should start with no listeners

Change-Id: I43211599b4d0369efba5cde426975475d563b891

java/com.sap.sse.security.userstore.mongodb/src/com/sap/sse/security/userstore/mongodb/UserStoreImpl.java

@@ -153,6 +153,7 @@ public class UserStoreImpl implements UserStore {
 
     @Override
     public void clear() {
+        listeners.clear();
         preferences.clear();
         preferenceConverters.clear();
         preferenceObjects.clear();