Improve streaming API

[clue]

Quite a few of the API endpoints take advantage of a streaming API.

We currently offer limited support for streaming incoming HTTP responses.

In the future, we should also look into streaming outgoing data even after having received the HTTP response header. This is relevant for attaching to read/write streams.

This depends on clue/reactphp-buzz#46.

[clue]

The improved readable streaming API has been implemented with #27.

In the future, we will also need to support sending streaming requests, see #33.