'Get updates between from_token and to_token. If Stream._LIMITED is
True then limit is provided, otherwise it\'s not.
Returns:
Deferred(list(tuple)): the first entry in the tuple is the token for
that update, and the rest of the tuple gets used to construct
a ``ROW_TYPE`` instance'

def update_function(self, from_token, current_token, limit=None):

    raise NotImplementedError()
