Add notmuch_thread_get_tags

[notmuch] / TODO

Write a "notmuch show" that displays a single thread.

Fix to use the *last* Message-ID header if multiple such headers are
encountered, (I noticed this is one thing that kept me from seeing the
same message-ID values as sup).

Fix "notmuch restore" to delete the old tags from a message/thread
before adding new ones. This will require someway to temporarily
'disconnect' a notmuch_message_t from the database, (that it, disable
automatic sync for add_tag, etc.), and then reconnect it. That is, the
removal and subsequent addition of tags to the message/thread needs to
be transactional.