Add next epoch finalization committee to block state hash

A hash of the next epoch finalization committee should be incorporated into the block state hash. This will be helpful for a future fast catch-up mechanism that avoids processing blocks. This involves (at least):
- [ ] Add the next epoch finalization committee hash to the block state. This should be updated when the next epoch bakers are updated.
- [ ] Incorporate the hash into the block state hash.

Note: these changes should be conditional on protocol >=P6.