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?