housekeeping in class loaders

SourceForge Submitter: rodburgett .
There are some housekeeping deficiencies among the
class loaders and repository; org.jboss.mx.loading
package. This patch adds some cleanup code and slight
refactoring.

The patch file for UnifiedClassLoader.java adds 2 clean
up lines to the unregister method; null the repository
reference and clear the classes container.

The patch file for UnifiedClassLoader3.java removes the
unregister method; the original method repeats cleanup
from the original super.unregister. Rather than make the
same change here as made in UnifiedClassLoader, this
updated class delegates all clean up to super.

The patch file for UnifiedRepositoryLoader3.java adds a
shutdown method to clean up and clear the container
fields.

The patch file for UnifiedRepositoryLoader3MBean.java
adds a shutdown method to expose the new shutdown
method in the loader. This method is exposed by the
class's mbean, changes are required elsewhere to make
use of this new method.

These change are not related to other, independent
changes in JBoss repository loader 3 have added a flush
method, and exposed it to JMX.

A separate patch to the mbean server implementation
will be posted with changes to invoke the new repository
shutdown method added by this patch.