Asynchronous pruning for RubyThreadPoolExecutor #574

Job	Run time
Test without C extension	3m 21s
Test isolated (2.3)	4m 4s
Tests: Ruby 2.3 - ubuntu	3m 36s
Test isolated (ruby)	3m 44s
Tests: Ruby 2.4 - ubuntu	3m 55s
Tests: Ruby 2.5 - ubuntu	4m 56s
Tests: Ruby 2.6 - ubuntu	3m 41s
Tests: Ruby 2.7 - ubuntu	3m 39s
Tests: Ruby 3.0 - ubuntu	3m 24s
Tests: Ruby 3.1 - ubuntu	3m 23s
Tests: Ruby 3.2 - ubuntu	3m 26s
Tests: Ruby 3.3 - ubuntu	3m 22s
Tests: Ruby 3.4 - ubuntu	3m 24s
Tests: Ruby jruby - ubuntu	8m 0s
Tests: Ruby truffleruby - ubuntu	4m 26s
Tests: Ruby ruby - windows	6m 51s
	1h 7m 12s

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