Created new SMB fixed header field: thread_id which contains the message number
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).