implement streaming responses ♫♫

One thing I'm unsure about here: is `byte[4096]` the optimal type for `HttpParser::buffer`? Perhaps another size or a dynamically growing buffer would be better.
Alas, this `streamRef` thing is actually important. The problem is that in the next line, we use `kj::mv(stream)` to move `stream` away. If we also do `stream->write(` in the same line, it is not defined which one happens first. GCC often does things in a different order from Clang.

(I guess I should add a comment every time I do this, but I do it so often...)