Asynchronous pruning for RubyThreadPoolExecutor #548

Job	Run time
Test without C extension	3m 18s
Test isolated (2.3)	2m 6s
Tests: Ruby 2.3 - ubuntu	3m 36s
Test isolated (ruby)	3m 45s
Tests: Ruby 2.4 - ubuntu	3m 40s
Tests: Ruby 2.5 - ubuntu	3m 43s
Tests: Ruby 2.6 - ubuntu	3m 43s
Tests: Ruby 2.7 - ubuntu	3m 39s
Tests: Ruby 3.0 - ubuntu	3m 24s
Tests: Ruby 3.1 - ubuntu	3m 12s
Tests: Ruby 3.2 - ubuntu	3m 19s
Tests: Ruby 3.3 - ubuntu	3m 15s
Tests: Ruby 3.4 - ubuntu	3m 13s
Tests: Ruby jruby - ubuntu	4m 49s
Tests: Ruby truffleruby - ubuntu	4m 12s
Tests: Ruby ruby - windows	7m 16s
	1h 0m 10s

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