Function send_goodbye 

pub(crate) async fn send_goodbye<S: Session>(
    session: &S,
    http: &Client,
    base_url: &str,
    snapshot: &FleetSnapshot,
    published_at_ms: u64,
    timeout: Duration,
) -> bool

Send the goodbye for snapshot (v3, gone): whether the relay stored it, which only a 2xx says (a 409 means it kept a newer, live record). Gives up after timeout. A relay that refuses v3 (an older one) gets nothing more: the record is never sent again as v1 or v2.