bench | tx-centrifuge: make cooldown configurable, default 0
Replace the hardcoded 300s worker cooldown with an optional top-level "cooldown_seconds" JSON field, defaulting to 0 (no wait). The cooldown was added to give multi-node benchmark clusters time to stabilise before traffic begins; for single-node ops / iterative testing it's a 5-minute tax with no benefit. Cluster runs opt back in explicitly. Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>