KIP-62 — Allow consumer to send heartbeats from a background thread

Accepted Kafka 1.0 Consumer

Moves consumer group heartbeat sending to a dedicated background `HeartbeatThread`, decoupling heartbeat liveness from the speed of the user's `poll()` processing loop. When message processing in a `poll()` loop takes longer than `session.timeout.ms`, the consumer misses heartbeats, causing a spurious rebalance even if the consumer is healthy and actively processing.

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