Asynchronous pruning for RubyThreadPoolExecutor #553

Job	Run time
Test without C extension	3m 18s
Test isolated (2.3)	2m 41s
Tests: Ruby 2.3 - ubuntu	4m 11s
Test isolated (ruby)	3m 34s
Tests: Ruby 2.4 - ubuntu	4m 13s
Tests: Ruby 2.5 - ubuntu	4m 17s
Tests: Ruby 2.6 - ubuntu	4m 23s
Tests: Ruby 2.7 - ubuntu	4m 13s
Tests: Ruby 3.0 - ubuntu	4m 0s
Tests: Ruby 3.1 - ubuntu	3m 56s
Tests: Ruby 3.2 - ubuntu	3m 18s
Tests: Ruby 3.3 - ubuntu	3m 20s
Tests: Ruby 3.4 - ubuntu	3m 16s
Tests: Ruby jruby - ubuntu	5m 16s
Tests: Ruby truffleruby - ubuntu	4m 12s
Tests: Ruby ruby - windows	6m 32s
	1h 4m 40s

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