Simplify miner deadline state to rely on cron every epoch

[anorth]

The miner actor maintains a deadline index in state, but the current deadline can also be inferred from the current epoch. The deadline exists in state because previously, when cron was not invoked for empty tipsets, the miner needed to track which deadline had been processed by cron (e.g. to detect a missed PoSt). With a reliable cron, the miner can instead assume that cron is for the "current" deadline. While this extra robusness isn't bad, it is confusing, especially to people not familiar with the old behaviour. I propose we remove the unnecessary complexity.

Related to #877, simplifying the power actor. After this, the power actor can reject events "in the past".

Doing this will make #658 redundant.

[ZenGround0]

Similarly we can rename ProvingPeriodStart to something like "FirstProvingPeriodStart" and stop updating it since only this value mod the ProvingPeriod is needed for fully deriving the current deadline.

Counter argument against doing this: it can be nice for more advanced users of miner state to look at these numbers to follow along with deadline progress. For example @acruikshank and I found them useful when debugging the CC upgrade and fault cron failures during space race.