KIP-1309 — Improve transaction liveness checking

Discussion TransactionsProtocolProducer

Introduces a TxnHeartbeat RPC (requiring WRITE on the Transactional ID) and a new InitProducerId version carrying a heartbeat interval, so the coordinator aborts idle transactions after a multiple of that interval rather than relying solely on transaction.timeout.ms. Transaction timeout currently conflates two goals — detecting dead clients and allowing long transactions to finish — so a low value aborts healthy transactions preemptively while a high value delays recovery from failed Flink, Spark or Streams applications.

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