Asynchronous pruning for RubyThreadPoolExecutor #581

Job	Run time
Test without C extension	3m 17s
Test isolated (2.3)	3m 47s
Tests: Ruby 2.3 - ubuntu	3m 28s
Test isolated (ruby)	3m 36s
Tests: Ruby 2.4 - ubuntu	3m 26s
Tests: Ruby 2.5 - ubuntu	3m 28s
Tests: Ruby 2.6 - ubuntu	3m 31s
Tests: Ruby 2.7 - ubuntu	3m 33s
Tests: Ruby 3.0 - ubuntu	3m 18s
Tests: Ruby 3.1 - ubuntu	3m 16s
Tests: Ruby 3.2 - ubuntu	3m 18s
Tests: Ruby 3.3 - ubuntu	3m 15s
Tests: Ruby 3.4 - ubuntu	3m 13s
Tests: Ruby jruby - ubuntu	9m 55s
Tests: Ruby truffleruby - ubuntu	4m 17s
Tests: Ruby ruby - windows	6m 3s
	1h 4m 41s

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