Function spend_showing 

async fn spend_showing<S>(
    socket: &mut ClientSocket<S>,
    state: &Arc<TransportState>,
    watching: &mut Watching,
    slot: &SessionSlot,
) -> Result<()>
where
    S: Sink<Message, Error = Error> + Unpin,

Spend what showing slot earns: its diamond, its failure mark and its death trace, and the home refresh a spent mark owes.

Called once per showing, from whichever half completes the pair.