tor-netdoc: ns preamble: break out validity_time_range (tidy)

impl Preamble {

    /// Calculate the validity range (time interval) for this network status document

    pub fn validity_time_range(&self) -> std::ops::Range<SystemTime> {

        let preamble = self;

        let lifetime = preamble.lifetime.clone();

        let delay = preamble.voting_delay.unwrap_or((0, 0));

        let lifetime = self.lifetime.clone();

        let delay = self.voting_delay.unwrap_or((0, 0));

        let dist_interval = time::Duration::from_secs(delay.1.into());

        let starting_time = *lifetime.valid_after - dist_interval;

        let timebound_range = starting_time..*lifetime.valid_until;

        timebound_range

        starting_time..*lifetime.valid_until

    }

}