Fix crashing http server when callback do not reply in place

Fix crashing http server when callback do not reply in place

General http callback looks like:
  static void http_cb(struct evhttp_request *req, void *arg)
  {
    evhttp_send_reply(req, HTTP_OK, "Everything is fine", NULL);
  }

And they will work fine becuase in this case http will write request
first, and during write preparation it will disable *read callback* (in
evhttp_write_buffer()), but if we don't reply immediately, for example:
  static void http_cb(struct evhttp_request *req, void *arg)
  {
    return;
  }

This will leave connection in incorrect state, and if another request
will be written to the same connection libevent will abort with:
  [err] ../http.c: illegal connection state 7

Because it thinks that read for now is not possible, since there were no
write.

Fix this by disabling EV_READ entirely. We couldn't just reset callbacks
because this will leave EOF detection, which we don't need, since user
hasn't replied to callback yet.

Reported-by: Cory Fields <cory@coryfields.com>