Get refresh token.

        :param client_name: A name one of configured clients
        :param redirect_uri: An URI for authorization redirect
        :returns: a coroutine

def refresh(self, client_name, refresh_token, **params):
        """Get refresh token.

        :param client_name: A name one of configured clients
        :param redirect_uri: An URI for authorization redirect
        :returns: a coroutine
        """
        client = self.client(client_name, logger=self.app.logger)
        return client.get_access_token(refresh_token, grant_type='refresh_token', **params)