nghttpx: Handle error from Downstream::resume_read()
nghttpx: Disable TLS renegotiation properly

4ed4efc does not disable TLS renegotiation at all, if client keeps
rengotiations without sending application data. In this change,
we intercept the raw incoming data from the client and if it is a
renegotiation, drop the connection immediately.
nghttpx: Take into account both frontend buffer and body buffer length
nghttpx: Rename ClientHandler::get_pending_write_length to get_outbuf_length

And other renames.
nghttpx: Move backend outbuf threshold to each downstream type
src: Use static_cast instead of reinterpret_cast if possible