Handle streaming-media responses

[benjamin-bader]

Proxied responses for e.g. video will chew up heap usage, and browsers will be unhappy.

We should sniff Content-Type and, if it's audio/ or video/, stop accumulating the body in memory and just ship it to the client.

This requires augmenting HttpMessageParser such that it can:

• [x] tell us when headers have all been parsed
• [ ] give us access to chunks of the response body as they are read

[benjamin-bader]

I'm not sure what I was thinking re: product features around streaming media. Current thoughts:

• we should not capture stream bodies by default - just shuffle the bytes as they come in.
• we should offer a config option to enable capturing full streaming responses

...we should test this with DRM'd streams like Netflix.