Asynchronous pruning for RubyThreadPoolExecutor #604

Job	Run time
Test isolated (ruby)	43s
Test without C extension	43s
Test isolated (2.3)	44s
Tests: Ruby 2.7 - ubuntu	44s
Tests: Ruby jruby - ubuntu	40s
Tests: Ruby 2.5 - ubuntu	44s
Tests: Ruby 3.0 - ubuntu	44s
Tests: Ruby 2.4 - ubuntu	42s
Tests: Ruby 3.2 - ubuntu	43s
Tests: Ruby 2.6 - ubuntu	44s
Tests: Ruby 3.3 - ubuntu	44s
Tests: Ruby 3.4 - ubuntu	44s
Tests: Ruby truffleruby - ubuntu	44s
Tests: Ruby 3.1 - ubuntu	44s
Tests: Ruby 2.3 - ubuntu	45s
Tests: Ruby ruby - windows	29s
	11m 21s

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