Function get_one_with_fallback 

pub async fn get_one_with_fallback<T, E: Display, F: Future<Output = Result<T, E>>>(
    replicas: &[(ReplicaId, SocketAddr)],
    rng: &mut EndRng,
    fallback_timeout: Duration,
    req: impl FnMut(SocketAddr) -> F,
) -> Option<T>