Function serve 

pub async fn serve<L, C, D>(
    listener: L,
    codec: Arc<C>,
    dispatch: Arc<D>,
    config: SessionConfig,
)
where
    L: Listener,
    C: EnvelopeCodec + 'static,
    D: Dispatch + 'static + ?Sized,

Accept connections from listener and serve each with run_session, bounded by SessionLimits::max_connections. The accept loop and all per-connection futures share one FuturesUnordered — spawn-free.