chanfeed: use get_all_threads instead of get_threads or loop pages

[nazunalika]

If a thread is being added that is not on page 1, there is a possibility it may be missed and an exception for the thread not existing will appear. We have not ran into this, but we are opening this just in case it does happen.

We need to determine if we can use get_all_threads or loop all pages for get_threads if the exception ever occurs.