Every request is now kept in a maximum of two queues. When a request is
initially inserted, it is inserted in the `to_write` queue. When at
least one write buffer is extracted from that request, it is *moved* to
the `wait_write` queue. When at least one byte of the request has been
written to the socket, it is *also* placed in the `wait_read` queue.
This means that it is possible for a request to be emitting write
buffers, while simultaneously receiving read callbacks.
