KIP-70 — Revise Partition Assignment Semantics on New Consumer's Subscription Change

Accepted Kafka 1.0 Consumer

Changes `KafkaConsumer` behavior on subscription changes (`subscribe`/`unsubscribe`) so that removed partitions have their offsets committed before the rebalance is triggered, rather than losing uncommitted offsets immediately. When `subscribe()` is called with a new topic list, partitions removed from the subscription lose any pending uncommitted offsets because the rebalance is triggered before the commit can happen.

Details