Fix div zero panic from zero fb calculation (#35)

[sieniven]

📝 Summary

This PR resolves the issue found on stress test where if the FCU arrives time=nice target_time-leeway_time, time drift calculated will be zero. This results in during calculate_flashblocks operation, the number of flashblocks to be built to be equal to zero, causing a panic on subsequent gas limit calculation due to a div-zero error.

The PR fixes handling on calculating time drift to additionally check if time drift is > 0.

Context of the issue:

✅ I have completed the following steps:

• [ ✅ ] Run make lint
• [✅ ] Run make test
• Added tests (if applicable)

[SozinM]

Let's do it like this?

        let time_drift = match target_time.duration_since(now) {
            Ok(time_drift) if !time_drift.is_zero() => time_drift,
            _ => {
                error!(
                    target: "payload_builder",
                    message = "FCU arrived too late or system clock are unsynced",
                    ?target_time,
                    ?now,
                );
                return (
                    self.config.flashblocks_per_block(),
                    self.config.specific.interval,
                );
            }
        };

[sieniven]

@SozinM Sure both fixes work. However, I think the original fix is more idiomatic since we are discarding the error entirely so unwrapping the result explicitly to an option and then doing the zero duration check might be better. I'll make the change to your suggestion if you think otherwise.

[SozinM]

@akundaz
@avalonche