And the web server crashing will cause the open streams to close, and new streams to fail to connect in the first place. But the actual circuit is not to Apache, it is to Tor, and it will stay open even if there is nothing listening to the traffic after Tor redirects it to a port after receiving it. For the actual circuit to abruptly close, it would mean that the remote instance of Tor crashed, Apache crashing will cause the streams to abruptly close but not the circuit.