Support HTTP 103 Early Hints

[thomasjm]

This PR follows on from snoyberg/http-client#524, to add support for HTTP 103 Early Hints to the reverse proxy.

The main change here is that we have to switch from WAI's responseStream response composer to responseRaw, because the nature of early hints means that we don't know the final HTTP status or headers up front, and the raw composer is the only one low-level enough to support sending (potentially multiple) HTTP 103 Early Hints sections before the main response.

This is slightly painful, because it

• Increases the code complexity slightly--we have to handle HTTP 1.1 chunking ourselves, because normally responseStream would do it.
• Requires a breaking change to the API, with defaultOnExc changing its signature. This is essentially because we have to stick the early hints callback on the request object before the call to responseOpen, and that callback needs to be able to start sending response data before responseOpen returns. As a result, I had to wrap the entire bracket call of waiProxyToSettings within the sendResponse call, which commits us to using responseRaw, which is why defaultOnExc has to work this way.

[snoyberg]

This is a level of code dupe I'm not comfortable with, at least as the maintainer of the project.