implement streaming requests

I think we'll need to have some kind of support for requests of unknown size. If we require that `expectSize()` be called, then the frontend proxy will need to buffer up any chunked requests it receives before forwarding them, which will probably involve placing a size limit on such requests, which somewhat defeats the purpose of having these streaming methods in the first place.

Do you think that approach is better than my current approach of not requiring `expectSize()` to be called?