Can we remove `nextProvingPeriod`?

[wjmelements]

I think a lot of it can be merged into possessionProven. I also wanted to remove provenThisPeriod.

[rvagg]

nextProvingPeriod is a requirement of PDPVerifier so we have to have something here, or propose to remove it there, but it exists separately for security reasons and to provide some amount of flexibility to the SP so they can optimise things. We can open that discussion up, but we'd be reaching back into original design decisions for PDP that are somewhat influenced by experience with the PoRep actor logic and the security & operational requirements of SP around that.
It would be interesting to explore opportunities to do batching in many of these PDP operations - an SP has the ability to move things around so that their proving periods line up for data sets, can we make their lives easier and cheaper by allowing batch submission? Is there any benefit in doing it at the contract layer vs multicall?