KIP-628 — ConsumerPerformance's multi-thread implementation

Discussion Consumer

Restores multi-thread support to `kafka-consumer-perf-test.sh` by implementing a `ConsumerPerfThread` class for the new consumer API, making the `--threads` option functional again. Multi-thread consumer performance testing was removed when the tool was migrated from the old consumer to the new consumer API, leaving no way to benchmark consumer throughput with multiple threads.

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