Only start an epoch instance if has synced to the current epoch and is part of the latest validator set

[samliok]

We currently start an epoch instance if it is a validator set of the transitioning epoch(i.e. the epoch onEpochChange is called on)
https://github.com/ava-labs/Simplex/blob/main/instance.go#L552

But what if the epoch is an old epoch? Maybe the validator set has changed since then and we aren't even a validator anymore.