UIMA Class loader to record if it has been closed

UIMA-5797 is implementing a close() call for UIMA Class loaders when the destroy() method is called.  UIMA-5801 needs to be able to tell if a UIMA Class Loader has been closed, so it can clean up a hashmap.  

Implement a bit that remembers closing, and provide an "isClosed()" method to query it.