Asynchronous pruning for RubyThreadPoolExecutor #608

Job	Run time
Test without C extension	49s
Test isolated (2.3)	47s
Test isolated (ruby)	48s
Tests: Ruby 2.4 - ubuntu	48s
Tests: Ruby 2.3 - ubuntu	48s
Tests: Ruby 2.5 - ubuntu	47s
Tests: Ruby 3.2 - ubuntu	49s
Tests: Ruby 2.6 - ubuntu	49s
Tests: Ruby 3.4 - ubuntu	49s
Tests: Ruby 3.3 - ubuntu	50s
Tests: Ruby 3.0 - ubuntu	49s
Tests: Ruby 2.7 - ubuntu	48s
Tests: Ruby truffleruby - ubuntu	49s
Tests: Ruby 3.1 - ubuntu	48s
Tests: Ruby jruby - ubuntu	47s
Tests: Ruby ruby - windows	34s
	12m 39s

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