Use appropriate defaults for threading options

[acogoluegnes]

Threading options to minimize the number of used threads for IoT workload can be involving. For a small IoT workload like the following

$ ./runjava com.rabbitmq.perf.PerfTest \
  --publishing-interval 5 --producer-random-start-delay 120 \
  --producers 100 --consumers 300 \
  --queue-pattern 'perf-test-%03d' \
  --queue-pattern-from 1 --queue-pattern-to 100

~ 1 K threads are used, which is OK. But for a bigger workload, e.g. 10 times more producers/consumers/queues, 10 K threads is too many. We can drastically reduce the number of used threads like the following:

$ ./runjava com.rabbitmq.perf.PerfTest \
  --publishing-interval 30 --producer-random-start-delay 300 \
  --producers 1000 --consumers 3000 \
  --queue-pattern 'perf-test-%04d' \
  --queue-pattern-from 1 --queue-pattern-to 1000
  --heartbeat-sender-threads 10 --nio-threads 20
  --consumers-thread-pools 100

Here ~ 150 threads are used, but the threading options aren't trivial.

PerfTest could come up with appropriate defaults for --heartbeat-sender-threads, --nio-threads, and --consumers-thread-pools as soon as --publishing-interval is used. These defaults could be overidden with the respective options in case they don't fit the workload.