Asynchronous pruning for RubyThreadPoolExecutor #558

Job	Run time
Tests: Ruby 2.3 - ubuntu	3m 59s
Tests: Ruby 2.4 - ubuntu	5m 10s
Test without C extension	3m 21s
Tests: Ruby 2.5 - ubuntu	4m 59s
Tests: Ruby 2.6 - ubuntu	3m 42s
Test isolated (2.3)	4m 57s
Tests: Ruby 2.7 - ubuntu	3m 39s
Test isolated (ruby)	3m 44s
Tests: Ruby 3.0 - ubuntu	3m 28s
Tests: Ruby 3.1 - ubuntu	3m 24s
Tests: Ruby 3.2 - ubuntu	3m 24s
Tests: Ruby 3.3 - ubuntu	3m 30s
Tests: Ruby 3.4 - ubuntu	3m 24s
Tests: Ruby jruby - ubuntu	10m 13s
Tests: Ruby truffleruby - ubuntu	4m 21s
Tests: Ruby ruby - windows	7m 52s
	1h 13m 7s

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