Asynchronous pruning for RubyThreadPoolExecutor #606

Job	Run time
Test without C extension	3m 13s
Test isolated (ruby)	3m 39s
Test isolated (2.3)	4m 3s
Tests: Ruby 2.4 - ubuntu	3m 26s
Tests: Ruby 2.3 - ubuntu	3m 38s
Tests: Ruby 2.6 - ubuntu	3m 32s
Tests: Ruby 2.5 - ubuntu	3m 30s
Tests: Ruby 2.7 - ubuntu	3m 32s
Tests: Ruby truffleruby - ubuntu	4m 13s
Tests: Ruby 3.3 - ubuntu	3m 19s
Tests: Ruby 3.2 - ubuntu	3m 23s
Tests: Ruby 3.0 - ubuntu	3m 15s
Tests: Ruby ruby - windows	6m 53s
Tests: Ruby jruby - ubuntu	5m 13s
Tests: Ruby 3.1 - ubuntu	3m 15s
Tests: Ruby 3.4 - ubuntu	3m 14s
	1h 1m 18s

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