Asynchronous pruning for RubyThreadPoolExecutor #562

Job	Run time
Test isolated (2.3)	4m 12s
Test without C extension	3m 24s
Test isolated (ruby)	3m 48s
Tests: Ruby 2.3 - ubuntu	3m 53s
Tests: Ruby 2.4 - ubuntu	4m 4s
Tests: Ruby 2.5 - ubuntu	4m 54s
Tests: Ruby 2.6 - ubuntu	3m 55s
Tests: Ruby 2.7 - ubuntu	3m 51s
Tests: Ruby 3.0 - ubuntu	3m 43s
Tests: Ruby 3.1 - ubuntu	3m 37s
Tests: Ruby 3.2 - ubuntu	3m 29s
Tests: Ruby 3.3 - ubuntu	3m 26s
Tests: Ruby 3.4 - ubuntu	3m 25s
Tests: Ruby jruby - ubuntu	10m 12s
Tests: Ruby truffleruby - ubuntu	5m 28s
Tests: Ruby ruby - windows	6m 42s
	1h 12m 3s

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