Asynchronous pruning for RubyThreadPoolExecutor #589

Job	Run time
Test without C extension	3m 12s
Test isolated (2.3)	4m 1s
Tests: Ruby 2.3 - ubuntu	3m 27s
Test isolated (ruby)	3m 34s
Tests: Ruby 2.4 - ubuntu	3m 24s
Tests: Ruby 2.5 - ubuntu	3m 36s
Tests: Ruby 2.6 - ubuntu	3m 31s
Tests: Ruby 2.7 - ubuntu	3m 28s
Tests: Ruby 3.0 - ubuntu	3m 17s
Tests: Ruby 3.1 - ubuntu	3m 17s
Tests: Ruby 3.2 - ubuntu	3m 14s
Tests: Ruby 3.3 - ubuntu	3m 17s
Tests: Ruby 3.4 - ubuntu	3m 12s
Tests: Ruby jruby - ubuntu	5m 18s
Tests: Ruby truffleruby - ubuntu	4m 15s
Tests: Ruby ruby - windows	5m 4s
	59m 7s

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