Add support for StreamingHttpResponse

[maurizi]

This is likely to be a better performing default for most people than a standard HttpResponse, but since StreamingHttpResponse is not a subclass of HttpResponse and has a different API, making it the default would be a breaking change (and we would add a non_streaming option to restore the old behaviour).

We could instead make it an optional feature by adding a render_to_streaming_csv_response function or adding an optional streaming option to render_csv_to_response.

See the Django docs for a general overview of StreamingHttpResponse and how to use it with CSVs