Asynchronous pruning for RubyThreadPoolExecutor #570

Job	Run time
Test without C extension	5m 17s
Test isolated (2.3)	1m 27s
Tests: Ruby 2.3 - ubuntu	6m 31s
Test isolated (ruby)	23s
Tests: Ruby 2.4 - ubuntu	7m 40s
Tests: Ruby 2.5 - ubuntu	8m 46s
Tests: Ruby 2.6 - ubuntu	6m 47s
Tests: Ruby 2.7 - ubuntu	6m 47s
Tests: Ruby 3.0 - ubuntu	6m 33s
Tests: Ruby 3.1 - ubuntu	6m 38s
Tests: Ruby 3.2 - ubuntu	5m 23s
Tests: Ruby 3.3 - ubuntu	5m 22s
Tests: Ruby 3.4 - ubuntu	5m 20s
Tests: Ruby jruby - ubuntu	7m 28s
Tests: Ruby truffleruby - ubuntu	6m 20s
Tests: Ruby ruby - windows	9m 28s
	1h 36m 10s

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