about strategy of socket closed

[huangwei1024]

HI,
when ClientSocket was closed, I think the Session._request_table were no longer necessary.
future under current strategy will be wait until timeout, but if Client set timeout None, when socket closed, there will be endless wait.

    def on_close(self, sock):
        # Avoid calling self.on_connect_failed after self.close called.
        if self._closed:
            return

        # if sock in self._sockets:
        #     self._sockets.remove(sock)
        #     self._connecting = 0
        # else:
        # Tornado does not have on_connect_failed event.
        self.on_connect_failed(sock)

What do you think?