Asynchronous pruning for RubyThreadPoolExecutor #565

Job	Run time
Test without C extension	3m 23s
Test isolated (2.3)	4m 24s
Tests: Ruby 2.3 - ubuntu	4m 21s
Test isolated (ruby)	3m 48s
Tests: Ruby 2.4 - ubuntu	4m 5s
Tests: Ruby 2.5 - ubuntu	3m 45s
Tests: Ruby 2.6 - ubuntu	3m 45s
Tests: Ruby 2.7 - ubuntu	3m 55s
Tests: Ruby 3.0 - ubuntu	3m 36s
Tests: Ruby 3.1 - ubuntu	3m 38s
Tests: Ruby 3.2 - ubuntu	3m 27s
Tests: Ruby 3.3 - ubuntu	3m 30s
Tests: Ruby 3.4 - ubuntu	3m 27s
Tests: Ruby jruby - ubuntu	4m 33s
Tests: Ruby truffleruby - ubuntu	4m 24s
Tests: Ruby ruby - windows	4m 18s
	1h 2m 19s

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