notmuch show: Implement proper thread ordering/nesting of messages.

notmuch show: Implement proper thread ordering/nesting of messages.

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).