Streaming responses without a known length #89

Every response today needs a length up front, including NGResponses.streaming( stream, length ) from #41. A response whose content is produced as it's sent can't be expressed: server-sent events, a generated export, a long download whose size isn't known in advance.

• Add a response type that streams until the stream ends, sent with chunked transfer encoding (HTTP/1.1) or without a length (HTTP/2).
• The adaptors must flush as content arrives, not buffer the whole response.
• Front ends can still buffer. SSE responses also send X-Accel-Buffering: no; see wo-adaptor-jetty's SSEStream.

This is the prerequisite for SSE in ng (#90).