Asynchronous pruning for RubyThreadPoolExecutor #544

Job	Run time
Test without C extension	3m 12s
Tests: Ruby 2.3 - ubuntu	6m 2s
Test isolated (2.3)	6m 4s
Tests: Ruby 2.4 - ubuntu	6m 1s
Test isolated (ruby)	3m 37s
Tests: Ruby 2.5 - ubuntu	6m 2s
Tests: Ruby 2.6 - ubuntu	6m 1s
Tests: Ruby 2.7 - ubuntu	6m 2s
Tests: Ruby 3.0 - ubuntu	6m 1s
Tests: Ruby 3.1 - ubuntu	6m 2s
Tests: Ruby 3.2 - ubuntu	3m 19s
Tests: Ruby 3.3 - ubuntu	3m 20s
Tests: Ruby 3.4 - ubuntu	3m 21s
Tests: Ruby jruby - ubuntu	6m 3s
Tests: Ruby truffleruby - ubuntu	6m 2s
Tests: Ruby ruby - windows	5m 51s
	1h 23m 0s

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