KIP-441 — Smooth Scaling Out for Kafka Streams

Accepted Kafka 2.5 StreamsConsumer

Introduces a `warmup` task state in Kafka Streams where a new instance restores standby changelogs before being assigned active tasks, so the prior active owner retains the active assignment until the new owner is sufficiently caught up. After a rebalance, stateful tasks assigned to a new instance could block processing for hours while rebuilding their state stores from scratch, causing severe availability degradation during scale-out.

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