Asynchronous pruning for RubyThreadPoolExecutor #603

Job	Run time
Test without C extension	28s
Test isolated (ruby)	29s
Test isolated (2.3)	17s
Tests: Ruby 2.4 - ubuntu	16s
Tests: Ruby 2.5 - ubuntu	15s
Tests: Ruby 2.3 - ubuntu	16s
Tests: Ruby 2.6 - ubuntu	16s
Tests: Ruby 3.0 - ubuntu	30s
Tests: Ruby jruby - ubuntu	18s
Tests: Ruby 2.7 - ubuntu	16s
Tests: Ruby truffleruby - ubuntu	29s
Tests: Ruby 3.3 - ubuntu	29s
Tests: Ruby 3.2 - ubuntu	29s
Tests: Ruby 3.4 - ubuntu	29s
Tests: Ruby ruby - windows	14s
Tests: Ruby 3.1 - ubuntu	27s
	5m 58s

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