Works together with 'mark_as_executed' and 'free' methods.

        It marks request as forwarded to 'to' replicas.
        To let request be removed, it should be marked as executed and each of
        'to' replicas should call 'free'.

def mark_as_forwarded(self, req: Request, to: int):
        """
        Works together with 'mark_as_executed' and 'free' methods.

        It marks request as forwarded to 'to' replicas.
        To let request be removed, it should be marked as executed and each of
        'to' replicas should call 'free'.
        """
        self[req.key].forwarded = True
        self[req.key].forwardedTo = to
        self[req.key].unordered_by_replicas_num = to