Asynchronous pruning for RubyThreadPoolExecutor #563

Job	Run time
Test without C extension	3m 27s
Tests: Ruby 2.3 - ubuntu	3m 45s
Test isolated (2.3)	4m 7s
Tests: Ruby 2.4 - ubuntu	4m 50s
Test isolated (ruby)	3m 51s
Tests: Ruby 2.5 - ubuntu	3m 46s
Tests: Ruby 2.6 - ubuntu	3m 45s
Tests: Ruby 2.7 - ubuntu	3m 44s
Tests: Ruby 3.0 - ubuntu	3m 31s
Tests: Ruby 3.1 - ubuntu	3m 33s
Tests: Ruby 3.2 - ubuntu	3m 24s
Tests: Ruby 3.3 - ubuntu	3m 28s
Tests: Ruby 3.4 - ubuntu	3m 24s
Tests: Ruby jruby - ubuntu	9m 40s
Tests: Ruby truffleruby - ubuntu	4m 27s
Tests: Ruby ruby - windows	5m 56s
	1h 8m 38s

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