Function retry 

pub fn retry<SLEEP, POL, F, Fut, T, E>(
    policy: POL,
    future_repeater: F,
) -> Retry<SLEEP, POL, T, E> 
where
    SLEEP: Sleepble + 'static,
    POL: RetryPolicy<E>,
    F: Fn() -> Fut + Send + 'static,
    Fut: Future<Output = Result<T, E>> + Send + 'static,