Fix admission control when an experiment has nodes already allocated.
(The original approach was reusing the existing experiment record when trying to repeat admission control. This did properly decrement the "used nodes" count, so that reallocating the same node to the same experiment wasn't counted twice; but unfortunately it didn't effectively increase the "free nodes" count and so it was still possible to encounter spurious failures. This could affect both swapmod and RedeemTicket.)
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