bench: what screening costs while catching up

Review 2, N7, measured first: a node whose document has no policy (a
joiner's empty one) applies what it receives unjudged. Screening it too
would cost finding every change's paths and judging it. bench_screening
takes 1 000, 10 000 and 100 000 changes into an empty document, as one
delta brings them: unjudged, as Automerge applies them, against
screened (access::screen::apply), with no policy in them (each passes,
read at its dependencies, after its paths were found) and with the
policy their first change wrote (each judged by the rules).

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