Remove the ShutdownXaDataSource.

Remove the ShutdownXaDataSource by letting the XaDataSourceManager.getXaDataSource
method throw an IllegalStateException is the manager is shut down, instead of
returning an instance of the ShutdownXaDataSource. Anyone who would now try
to get an XaDataSource from a manager that has been shut down, will now fail
earlier than before, and with a more explicit exception and a more local
stack trace. I think it's a good change, but it is none the less a change in
behaviour. However, this is in kernel/impl, so no one besides us should depend
on it. Also, no tests failed because of this change, as far as I can tell.
I have mixed feelings about that fact.