Use out-of-band buffers for Python buffer protocol supporting objects

For objects supporting [Python's buffer protocol]( https://docs.python.org/3.8/c-api/buffer.html ) (like `memoryview`), it would be great to have a path where we use [Pickle's protocol 5 for serialization of out-of-band buffers]( https://docs.python.org/3.8/library/pickle.html#out-of-band-buffers ). This should avoid some copies when pickling/unpickling and allow more efficient transmission.