of the first message in the thread, or 0 if unknown (e.g. legacy message). When
a new message is created, the thread_id value of the message being replied-to
will be used, else if the new msg is not a reply, then the new message's number
will be the value used for the thread_id.
This "thread-id" feature (Deuce's idea) allows threads with a common parent to
remain logically grouped after the original messages in the thread have been
purged (though the original hiearchy will start to be lost).