This reduces our reliance on open message_file objects, (so is a step
toward fixing the "too many open files" bug), but more importantly, it
means we don't load a self-referencing in-reply-to header, (since we
weed those out before adding any replyto terms to the database).
We now properly analyze the in-reply-to headers to create a proper
tree representing the actual thread and present the messages in this
correct thread order. Also, there's a new "depth:" value added to the
"message{" header so that clients can format the thread as desired,
(such as by indenting replies).
The existing notmuch_message_get_header is *almost* good enough for
this, except that we also need to remove the '<' and '>'
delimiters. We'll probably want to implement this function with
database storage in the future rather than loading the email message.
