Support out-of-band buffers in Python pickling

[jakirkham]

When Python pickle's protocol 5 or greater is used, this change will support more efficient serialization of out-of-band buffers. This is analogous to Dask's custom serialization except for pickling. As such this is helpful in any Python serialization case where pickling is used. If an older pickling protocol is used, we simply proceed as before.

Also as we have switched from __getstate__/__setstate__ to __reduce_ex__, we no longer need to build a DeviceBuffer and then steal its unique_ptr for our own DeviceBuffer (which pickle also constructs). Instead we can just build a DeviceBuffer once and return it. So this is a bit more efficient.

Note this is analogous to the change made to Buffer ( rapidsai/cudf#5132 ) except for DeviceBuffer.

[jakirkham]

cc @kkraus14 @shwina