KIP-211 — Revise Expiration Semantics of Consumer Group Offsets

Accepted Kafka 2.1 Consumer

Changes offset expiration semantics so that committed offsets for a consumer group are only expired after both `offsets.retention.minutes` has elapsed since the last commit AND the consumer group itself is empty (no active members). Under the old semantics, offsets for inactive partitions within an otherwise active group expired on their own schedule, causing active consumers to lose their committed position and reset to `auto.offset.reset`.

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