Function start_drain_incoming_ack_pipeline 

async fn start_drain_incoming_ack_pipeline<AckIn>(ack_incoming: AckIn)
where
    AckIn: Stream<Item = (OffchainPublicKey, Vec<Acknowledgement>)> + Send + 'static,

Drains incoming acknowledgements without forwarding them to an [UnacknowledgedTicketProcessor].

Used by Entry and Exit nodes — neither processes incoming ticket acknowledgements. Entry nodes receive acks from relays (they pay for forwarding), Exit nodes keep the pipeline alive for future PIX use. In both cases the queue must be actively drained; dropping the receiver causes every inbound ack dispatch to fail with SendError(disconnected).