Asynchronous pruning for RubyThreadPoolExecutor #591

Job	Run time
Test isolated (2.3)	3m 59s
Tests: Ruby 2.3 - ubuntu	3m 33s
Test isolated (ruby)	3m 41s
Test without C extension	3m 13s
Tests: Ruby 2.4 - ubuntu	3m 26s
Tests: Ruby 2.5 - ubuntu	3m 30s
Tests: Ruby 2.6 - ubuntu	3m 35s
Tests: Ruby 2.7 - ubuntu	3m 29s
Tests: Ruby 3.0 - ubuntu	3m 12s
Tests: Ruby 3.1 - ubuntu	3m 20s
Tests: Ruby 3.2 - ubuntu	3m 14s
Tests: Ruby 3.3 - ubuntu	3m 16s
Tests: Ruby 3.4 - ubuntu	3m 13s
Tests: Ruby jruby - ubuntu	5m 17s
Tests: Ruby truffleruby - ubuntu	4m 14s
Tests: Ruby ruby - windows	5m 49s
	1h 0m 1s

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