We no longer override Danga::Socket::event_write and instead
re-enable reads by queuing up another callback in the $close
response callback. This is necessary because event_write may not be
completely done writing a response, only the existing buffered data.
Furthermore, the {closed} field can almost be set at any time when
writing, so we must check it before acting on pipelined requests as
well as during write callbacks in more().