Asynchronous pruning for RubyThreadPoolExecutor #551

Job	Run time
Test isolated (2.3)	2m 12s
Test without C extension	3m 13s
Test isolated (ruby)	3m 34s
Tests: Ruby 2.3 - ubuntu	3m 42s
Tests: Ruby 2.4 - ubuntu	3m 46s
Tests: Ruby 2.5 - ubuntu	3m 42s
Tests: Ruby 2.6 - ubuntu	3m 45s
Tests: Ruby 2.7 - ubuntu	3m 41s
Tests: Ruby 3.0 - ubuntu	3m 33s
Tests: Ruby 3.1 - ubuntu	3m 23s
Tests: Ruby 3.2 - ubuntu	3m 17s
Tests: Ruby 3.3 - ubuntu	3m 17s
Tests: Ruby 3.4 - ubuntu	3m 15s
Tests: Ruby jruby - ubuntu	5m 3s
Tests: Ruby truffleruby - ubuntu	4m 10s
Tests: Ruby ruby - windows	4m 55s
	58m 28s

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