MapExternalizer no longer needs to extend InstanceReusingAdvancedExternalizer

The MapExternalizer originally extended InstanceReusingAdvancedExternalizer to allow for the CacheStatusResponse class to properly reuse values. However with ISPN-3561 we no longer use just a Map but also a ManagerStatusResponse so we only need the Externalizer for that class to extend InstanceReusingAdvancedExternalizer.