Function fetch_chunks 

pub async fn fetch_chunks(
    fud: &Fud,
    hash: &Hash,
    chunked: &mut ChunkedStorage,
    seeders_sub: &Subscription<Option<Vec<FudSeeder>>>,
    favored_seeder: Option<FudSeeder>,
    chunks: &mut HashSet<Hash>,
) -> Result<()>

Fetch chunks for chunked (file or directory) from seeders in seeders_sub.