Only read through payload on verify if actually needed
    
    If none of our verify items ranges over the payload, then why bother?
    
    To do this, add an internal rpmvs API to get it's range, and use
    that to decide whether trip over the payload is needed or not.
    In addition, the payload digest tag needs to be grabbed outside of the
    condition to avoid depending on other values. The details including
    RPMVSF_NEEDPAYLOAD will be handled internally to rpmvs which makes it
    actually nicer code-wise too.