Asynchronous pruning for RubyThreadPoolExecutor #552

Job	Run time
Tests: Ruby 2.3 - ubuntu	3m 56s
Test without C extension	3m 12s
Test isolated (2.3)	2m 17s
Tests: Ruby 2.4 - ubuntu	3m 58s
Test isolated (ruby)	3m 34s
Tests: Ruby 2.5 - ubuntu	3m 53s
Tests: Ruby 2.6 - ubuntu	3m 49s
Tests: Ruby 2.7 - ubuntu	3m 52s
Tests: Ruby 3.0 - ubuntu	3m 35s
Tests: Ruby 3.1 - ubuntu	3m 28s
Tests: Ruby 3.2 - ubuntu	3m 12s
Tests: Ruby 3.3 - ubuntu	3m 15s
Tests: Ruby 3.4 - ubuntu	3m 18s
Tests: Ruby jruby - ubuntu	5m 2s
Tests: Ruby truffleruby - ubuntu	4m 14s
Tests: Ruby ruby - windows	7m 14s
	1h 1m 49s

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