Function start 

pub async fn start<UD, SL, SR, IR>(
    cancellation_token: CancellationToken,
    listener: TcpListener,
    underlay_discovery: UD,
    segment_lister: SL,
    snap_resolver: SR,
    identity_registry: Arc<IR>,
    token_verifier: SnapTokenVerifier,
    metrics: Metrics,
) -> Result<()>
where
    UD: UnderlayDiscovery + 'static + Send + Sync,
    SL: SegmentsDiscovery + 'static + Send + Sync,
    SR: SnapDataPlaneResolver + 'static + Send + Sync,
    IR: SnapTunIdentityRegistry + 'static + Send + Sync,

Start the SNAP control plane API server.