feat: drand: refactor round verification

[arajasek]

Related Issues

This is a refactor of some of the beacon logic in preparation of FIP-0063, although this doesn't actually implement any logic towards that.

We currently assume that the beacon entries included in blocks must make a contiguous chain (no skipped rounds). This is currently needed, because we're using chained randomness.

Proposed Changes

This PR doesn't lead to any change in behaviour, but changes the logic to instead rely on expected rounds. We expect a certain round for every epoch, and include the beacon entry corresponding to that round in our blocks.

This makes the logic easier to extend when we switch to unchained randomness with a different period than Filecoin.

Additional Info

Checklist

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[Stebalien]

Suggested change

	for currEpoch > parentEpoch {
	for currEpoch := epoch; currEpoch > parentEpoch; curEpoch-- {