Asynchronous pruning for RubyThreadPoolExecutor #602

Job	Run time
Test without C extension	3m 15s
Test isolated (ruby)	3m 42s
Test isolated (2.3)	4m 0s
Tests: Ruby 2.6 - ubuntu	3m 31s
Tests: Ruby 2.3 - ubuntu	3m 33s
Tests: Ruby 2.4 - ubuntu	3m 36s
Tests: Ruby 3.1 - ubuntu	3m 15s
Tests: Ruby jruby - ubuntu	5m 3s
Tests: Ruby ruby - windows	6m 27s
Tests: Ruby 3.2 - ubuntu	3m 19s
Tests: Ruby 3.4 - ubuntu	3m 16s
Tests: Ruby 3.3 - ubuntu	3m 27s
Tests: Ruby 3.0 - ubuntu	3m 16s
Tests: Ruby 2.7 - ubuntu	3m 29s
Tests: Ruby truffleruby - ubuntu	4m 27s
Tests: Ruby 2.5 - ubuntu	3m 27s
	1h 1m 3s

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