Fix #1320: fix handling of futures in work-in-progress check

[leplatrem]

This PR:

• fixes a bug where signed collections would show up in info (because zipping two lists of different sizes)
• does not compare records if status is to-review (no need)
• does not fetch records in parallel anymore (since old wip collections are rare, no need)

[alexcottner]

Longer term - the kinto API should be flexible enough to return exactly the data that clients need. This app really shouldn't need to have business logic in it. If clients need to do further filtering of data that feels like a miss in our API tier.