Kill established session threads when we kill SMB server

[bwatters-r7]

While testing #18680, I saw that if you kill all the services, the thread listening on port 445 goes away, but the threads serving the established connections do not. If I establish an SMB session, then kill all the smb servers, nothing is listening on 445, but I can still download and interact with the share on an established connection, even though it says no services are there.
This makes sense if we are rigidly defining the service as the listener, which I could see, but it means that the servicing threads will continue to run well after the service is gone; we just won't accept new connections.

See #18680 (comment)