Consider using `Concurrent.available_processor_count` to set default threads count

[pjurewicz]

In the containerized environment, calling Concurrent.processor_count gives us host machine processor count.
The proper question to ask is how many CPU cores are usable and now we can do that by calling Integer(Concurrent.available_processor_count).
It would require sticking to the 'concurrent-ruby', '>= 1.3.1'.
It should help avoid exceeding database connection pools and slowing down job execution time for many people using K8s.