calculate reclaim_interval based on takenUntil

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Currently we have a fixed reclaim_interval seconds. In this thread, Tom's suggesting we should follow docker- and generic-worker's examples in calculating when to reclaim, using on some decimal multiplier or seconds before takenUntil.

When we do this, we should also look into modifying the notarization-poller in scriptworker-scripts.

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We should take bug 1611255 into account if/when we do this. Our current approach doesn't run as much risk of accidental claim-expired.

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Aiui, we went with 5 minutes hardcoded because a) it's simpler than calculating against takenUntil, b) unless takenUntil changes significantly, it's less likely to hit claimExpired due to slow reclaims, and c) scriptworkers have thus far been significantly smaller in quantity than our docker- and generic-worker pools. I'm leaning towards leaving it, but leaving this issue open in case we want to revisit.