Miners should not have power while late submitting their PoSts

A simple fix here is to return 0 in minerActor.GetPower() if currentHeight > miner.ProvingPeriodEnd

However, this raises the question of what happens if everyone is late on their PoSts somehow. Its not too difficult to think of what to do though, one option would be to just fork from before the point where everyone was late, and mine from there.

Start at minerActor.GetPower and trace how currentHeight and ProvingPeriodEnd determine a miner's power during late PoSt submissions. Resolve the stated all-miners-late scenario before changing the specification; done means the expected power and chain behavior are explicitly defined.

Written by the indexing model from the issue text.