Represents the top level node, i.e. persistence node in persistence.xsd.
Since this is a top level node, it extends {@link BundleNode}. This class
registers a handler {@link PersistenceNode} which is responsible for reading
the persistence-unit elements.
This is the default constructor which is also called from other
constructors of this class. Inside this constructor, we clear the
handlers set up by super classes' constructors because they are
not applicable in the context of PersistenceNode because
unlike standard Java EE schemas, persistence.xsd does not include
javaee_5.xsd for things like description, version etc.
// clear all the handlers set up by super classes.
if (handlers != null) handlers.clear();
registerElementHandler(
new XMLElement(PersistenceTagNames.PERSISTENCE_UNIT),
PersistenceUnitNode.class);
