Function queue_once 

pub fn queue_once(
    talk: &mut Talk,
    store: &Talks,
    text: &str,
    key: &str,
) -> Result<bool>

queue for a message that must be accepted at most once: key is recorded in Talk::notified in the same write as the draft, so a retry after a crash (or a second sweep) finds the key and queues nothing. Returns whether this call queued it.