Asynchronous pruning for RubyThreadPoolExecutor #572

Job	Run time
Test without C extension	22s
Test isolated (2.3)	9s
Tests: Ruby 2.3 - ubuntu	10s
Test isolated (ruby)	22s
Tests: Ruby 2.4 - ubuntu	9s
Tests: Ruby 2.5 - ubuntu	10s
Tests: Ruby 2.6 - ubuntu	10s
Tests: Ruby 2.7 - ubuntu	10s
Tests: Ruby 3.0 - ubuntu	22s
Tests: Ruby 3.1 - ubuntu	22s
Tests: Ruby 3.2 - ubuntu	44s
Tests: Ruby 3.3 - ubuntu	23s
Tests: Ruby 3.4 - ubuntu	22s
Tests: Ruby jruby - ubuntu	9s
Tests: Ruby truffleruby - ubuntu	1s
Tests: Ruby ruby - windows	11s
	4m 16s

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