WHAT

Kafka broker default the config `num.recovery.threads.per.data.dir` to 1, which means it will only have one thread to do log recovery, and let other CPUs idle. Because during log recovery, the broker basically does nothing, using only 1 thread for log recovery is wasteful and impact the performance.

WHY

Improve the log recovery performance 

HOW

Run benchmark tests to find out the best configuration for RHOSAK

DONE

1. Add the new `num.recovery.threads.per.data.dir` number as the default broker config in fleetshard
2. Document the findings, ex: the performance gains