Currently, mixing futures and streaming_callback does not guarantee
ordering (so the returned future could become ready before the last
streaming_callback fires). Specifically, this happens with
test_streaming_read_until_close_after_close if that test is modified
to not use the callback argument. This would cause problems when we
remove the callback argument if we left streaming_callback in place.

These problems are solvable, but probably not worth it since
partial=True is an alternative (and used internally instead of
streaming_callback).