Engine namespace cleanup

This creates a `cleanup.py` module and registers `cleanup.cleanup_all` with `atexit`.  The thinking is that when the client process shuts down, all distarray-related objects should be removed from the engines.  We tried doing this with a `Context.__del__` method, but due to the lack of guarantees surrounding `__del__` methods, it wasn't always called.  The `cleanup_all()` function wipes everything out, and should be safe.

This probably makes the `dump()` and `purge()` commands obsolete, since I never see any distarray-related objects on the engines anymore after the testsuite is finished running.