screen: read once per batch whether the policy is strict

The profile of bench_screening puts screening at about 21% of the
samples, half of it Automerge applying the changes. Of the other half,
reading the policy for every change taken is the largest part (3.4%):
reaches_signed reads it, and what users sign, to tell whether the
change must go in before the next is judged.

Read them once, and again only after the document took the accepted
ones; a change to the policy or to what users sign is relevant, taken
at once. A change touching nothing reaches nothing.

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