Add a ``user_id`` for the given ``address``.

        Implicitly adds the address if it was unknown before.

def add_userid_for_address(self, address: Address, user_id: str):
        """ Add a ``user_id`` for the given ``address``.

        Implicitly adds the address if it was unknown before.
        """
        self._address_to_userids[address].add(user_id)