Deep copying of Python objects backed by the object store

[pcmoritz]

If we do a deep copy of a Python object (like a numpy array) whose memory is backed by the object store, it might happen that the Destructor object (RayDealloc in worker.py) is copied, but might keep the same handle and segmentid. This would result in the object getting unmapped twice! Similar problems might happen with the object reference that is stored in the object upon "pull". We should investigate this more.