Asynchronous pruning for RubyThreadPoolExecutor #538

Job	Run time
Test without C extension	3m 18s
Tests: Ruby 2.3 - ubuntu	10m 14s
Test isolated (2.3)	6m 30s
Tests: Ruby 2.4 - ubuntu	10m 14s
Test isolated (ruby)	3m 46s
Tests: Ruby 2.5 - ubuntu	10m 14s
Tests: Ruby 2.6 - ubuntu	10m 12s
Tests: Ruby 2.7 - ubuntu	10m 13s
Tests: Ruby 3.0 - ubuntu	10m 14s
Tests: Ruby 3.1 - ubuntu	10m 13s
Tests: Ruby 3.2 - ubuntu	3m 14s
Tests: Ruby 3.3 - ubuntu	3m 21s
Tests: Ruby 3.4 - ubuntu	3m 15s
Tests: Ruby jruby - ubuntu	2m 15s
Tests: Ruby truffleruby - ubuntu	4m 14s
Tests: Ruby ruby - windows	4m 45s
	1h 46m 12s

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