SH-3184/SH-3221 Improve cleanup, destructor, thread termination, etc. logic in library.

With this commit, the cleanup paths should be production quality.  Unit tests have been
expanded to include cases requiring thread termination and cleanup by the worker thread.
Special operation/request added to support the unit tests.  Thread interface expanded
to include a very aggressive cancel() method that does not do cleanup but prevents the
thread from accessing objects that will be destroyed.