Asynchronous pruning for RubyThreadPoolExecutor #568

Job	Run time
Test without C extension	3m 24s
Test isolated (2.3)	4m 11s
Tests: Ruby 2.3 - ubuntu	4m 42s
Test isolated (ruby)	3m 47s
Tests: Ruby 2.4 - ubuntu	4m 15s
Tests: Ruby 2.5 - ubuntu	4m 6s
Tests: Ruby 2.6 - ubuntu	3m 43s
Tests: Ruby 2.7 - ubuntu	3m 47s
Tests: Ruby 3.0 - ubuntu	3m 30s
Tests: Ruby 3.1 - ubuntu	3m 33s
Tests: Ruby 3.2 - ubuntu	3m 30s
Tests: Ruby 3.3 - ubuntu	3m 25s
Tests: Ruby 3.4 - ubuntu	3m 23s
Tests: Ruby jruby - ubuntu	9m 24s
Tests: Ruby truffleruby - ubuntu	4m 25s
Tests: Ruby ruby - windows	6m 5s
	1h 9m 10s

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