Not all functions operating on party_weights are safe for sum of weights > u64::MAX

[matteojug]

Not all functions operating on party_weights are safe for sum of weights > u64::MAX:

#[tokio::test]
async fn test_ballot_max_weight() {
    let weights = vec![u64::MAX, 1];
    let threshold = BPConConfig::compute_bft_threshold(weights.clone());
    let cfg = BPConConfig::with_default_timeouts(weights, threshold);

    let (parties, receivers, senders) = create_parties(cfg);
    let ballot_tasks = launch_parties(parties);
    let p2p_task = propagate_p2p(receivers, senders);
    let results = await_results(ballot_tasks).await;
    p2p_task.abort();

    analyze_ballot(results);
}