Put killers in helper threads

[codecov[bot]]

Codecov Report

Merging #174 (b846f8d) into master (59e340e) will not change coverage. The diff coverage is 100%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #174 +/- ## ===================================== Coverage 86% 86% ===================================== Files 31 31 Lines 4091 4091 Branches 4091 4091 ===================================== Hits 3482 3482 Misses 591 591 Partials 18 18 ```

[github-actions[bot]]

Against v1.4.0 [hash=64; threads=1]: 🆗 Elo difference: 27.85 +/- 23.36, LOS: 99.03 %, DrawRatio: 29.67 %

[github-actions[bot]]

Against master [hash=64; threads=1]: 🆗 Elo difference: -15.65 +/- 22.22, LOS: 8.38 %, DrawRatio: 36.17 %

[github-actions[bot]]

Against v1.4.0 [hash=256; threads=4]: 🆗 Elo difference: 12.17 +/- 38.48, LOS: 73.27 %, DrawRatio: 36.50 %

[github-actions[bot]]

Against master [hash=256; threads=4]: 🆗 Elo difference: -17.39 +/- 37.09, LOS: 17.86 %, DrawRatio: 41.00 %