Asynchronous pruning for RubyThreadPoolExecutor #573

Job	Run time
Test without C extension	3m 28s
Test isolated (2.3)	1m 8s
Tests: Ruby 2.3 - ubuntu	3m 49s
Test isolated (ruby)	3m 46s
Tests: Ruby 2.4 - ubuntu	3m 38s
Tests: Ruby 2.5 - ubuntu	4m 12s
Tests: Ruby 2.6 - ubuntu	3m 45s
Tests: Ruby 2.7 - ubuntu	3m 45s
Tests: Ruby 3.0 - ubuntu	3m 29s
Tests: Ruby 3.1 - ubuntu	3m 36s
Tests: Ruby 3.2 - ubuntu	3m 23s
Tests: Ruby 3.3 - ubuntu	3m 24s
Tests: Ruby 3.4 - ubuntu	3m 27s
Tests: Ruby jruby - ubuntu	9m 36s
Tests: Ruby truffleruby - ubuntu	4m 29s
Tests: Ruby ruby - windows	5m 58s
	1h 4m 53s

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