notmuch_message_get_message_id: Fix to cache result

notmuch_message_get_message_id: Fix to cache result

Previously, this would allocate new memory with every call. That
was with talloc, of course, so there wasn't any leaking (eventually).
But since we're now calling this internally we want to be a little
less wasteful. It's easy enough to just stash the result into the
message on the first call, and then just return that on subsequent
calls.