Function verify_request 

pub fn verify_request(
    req: &LicenceRequest,
    steward_pubkey: &[u8; 32],
    steward_node: &str,
    audience: &str,
    now_ms: u64,
    replay: &mut ReplayGuard,
) -> Result<(), RequestRefused>

Verify req against the bound steward. Cheap checks first; the signature last; only a request whose signature verified is charged a nonce.