Function release 

pub(crate) async fn release(
    base: &str,
    agent_id: &str,
    token: &str,
    http: &Client,
    still_gone: impl Fn() -> bool,
) -> bool

Release agent_id’s lease (best effort) and forget it — unless still_gone says it was subscribed again by the time this runs, in which case this returns false (it did not let go).