notmuch-index-message: Start generating correct thread_id values.

notmuch-index-message: Start generating correct thread_id values.

Currently we're looking up all parents (based on In-reply-to and
References header) and using the list of all thread_id values
from those as our thread_id value. We're missing one step which
sup does which is to also look up any children in the database
that have reference our message ID. So we'll need to do that next.