The archive-thread race condition doesn't even exist now because there's
no command for modifying tags at the level of a thread (just individual
messages).
This means that the restore operation will now properly pick up the
removal of tags indicated by the tag just not being present in the
dump file.
We added a few new public functions in order to support this:
notmuch_message_freeze
notmuch_message_remove_all_tags
notmuch_message_thaw
