[FLIP 204] Add TargetEndTime to FlowEpoch smart contract

[jordanschalm]

Addresses #394.

This PR adds the new field TargetEndTime to the EpochSetup event, which will be used by the Cruise Control system to moderate block rate so that epochs end at the desired time. See details in FLIP 204

I have not added targetEndTime to the EpochStart event. Here is why:

• We can't easily persist the target end time, without creating a new ad-hoc storage path. I don't think this is worth the extra complexity.
• The timing config can change between the EpochSetup and EpochStart events. If we don't store the value, we may communicate different values in different events for the same epoch.

[joshuahannan]

Looks good! I'll push back the dates on the upgrades so we can get this in

[joshuahannan]

Will every epoch always have the same duration? I understand the config can change, but what if the duration changes, but we are using a reference epoch that is before the change? Would we just never do that?

[jordanschalm]

Right now, all epochs have the same duration (1 week on Mainnet). If/when we want to change the duration, then we would change the timing config.

The reference epoch needs to contain a real reference counter that has been used in the past. But the associated duration can be anything we want (it does not need to match the duration that the reference epoch actually took, when it occurred). So there isn't anything stopping us from using a reference epoch that is before the duration change, if we want to.

[jordanschalm]

I'll push back the dates on the upgrades so we can get this in

👍 Initially I thought we would need to push a flow-go patch before including this change in an upgrade. However, it seems that the relevant service event parsing is already backward-compatible (tested in onflow/flow-go#5013), so we won't need to do this.

We should still make sure to go through an epoch transition on Canary before upgrading Testnet/Mainnet to be safe, but otherwise there's no blockers for including this in the next upgrade.