http: chunk in the server, not middleware

http: chunk in the server, not middleware

Since PSGI does not require Transfer-Encoding: chunked or
Content-Length, we cannot expect random apps we host to chunk
their responses.

Thus, to improve interoperability, chunk at the HTTP layer like
other PSGI servers do.  I'm chosing a more syscall-intensive method
(via multiple send(...MSG_MORE) for now to reduce copy + packet
overhead.