Don't sign over the PartSetHeader or include it in the Header

[evan-forbes]

Summary

As discussed in the comments here and here, we should not include the PartSetHeader in the data that is signed over or used to create the hash of the Header. While we're at it, we might as well remove the LastPartSetHeader field from Header completely after #441 is merged.

Details

We're planning on only using the header hash for the BlockID #184, which means that the PartSetHeader only needs to used when propagating blocks during consensus (but that might change too, #434) and should not be in the header. If it's not in the header, then the hash PartSetHeader can't be validated. Therefore, it should not contribute to the hash or be signed over.

Action Items

• Stop including the PartSetHeader in the bytes that are signed over when voting
• Remove the LastPartSetHeader field from the Header (and thus removing from contributing to the hash)

References

ref Define commit validation #92

part of #418

blocked by #441