fix: support multiple partitions in PoRep

[th7nder]

Description

Fixes #747.
When a PoRep is configured with multiple partitions (meaning many more challenges to prove) we didn't support this case.
It's still untested, as we don't have configuration with those multiple partitions.
It'll be followed-up in #751.

Checklist

• Are there important points that reviewers should know?
  • If yes, which ones?
• Make sure that you described what this change does.
• If there are follow-ups, have you created issues for them?
  • If yes, which ones? / List them here
    • Adjust production values for challenges with 1GiB sector #751
• Have you tested this solution?
• Were there any alternative implementations considered?
• Did you document new (or modified) APIs?

[jmg-duarte]

lgtm