KIP-540 — Implement per key stream time tracking

Discussion Streams

Proposes per-key stream time tracking in Kafka Streams, where stream time advances independently for each key rather than as a single high-watermark across all records in a partition. The global per-partition stream time model incorrectly advances time for all keys when any key's record has a high timestamp, causing premature window closing and grace period expiry for keys whose own data is late or sparse.

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