notmuch restore: Fix to remove all tags before adding tags.

notmuch restore: Fix to remove all tags before adding tags.

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