fix: PoSt partition batching

[th7nder]

Description

Partition Index in PoSt generation was unused, I thought that's an issue and it's not.
k is local to each pair of the verification/generation batch.
If we have 20 partitions to prove, two batches of 10 partitions, k = 0..10 in each batch.

Anyways, this PR reduces the PoSt partition batches to 1 per call, as this is max we can verify with our partition sizes given the block time. I changed the constants to reflect that.

Closes #787.

Checklist

• Are there important points that reviewers should know?
  • If yes, which ones?
• Make sure that you described what this change does.
• Have you tested this solution?
• Were there any alternative implementations considered?
  • Yes, we could make the partition index be consistent with the 'global partition index' stored in the SP deadline, but after analysis I deem those changes too big and not worth it.
• Did you document new (or modified) APIs?