Set default parallelism for stage operators to the available number of processors

[sundargates]

Context

For stages that can be executed in parallel, set the default parallelism to the available number of cores.

Checklist

• [ ] ./gradlew build compiles code correctly
• [ ] Added new tests where applicable
• [ ] ./gradlew test passes all tests
• [ ] Extended README or added javadocs where applicable

[github-actions[bot]]

Test Results

520 tests  ±0   514 :white_check_mark: ±0   7m 24s :stopwatch: -43s 135 suites ±0     6 :zzz: ±0  135 files   ±0     0 :x: ±0 

Results for commit 656c7ef3. ± Comparison against base commit d773089b.

:recycle: This comment has been updated with latest results.