Merge pull request #1192 from herwinw/thread_group #1736

Job	Run time
RuboCop	26s
specs (ubuntu, 3.0.6)	1m 31s
specs (ubuntu, 3.1.4)	1m 29s
specs (ubuntu, 3.2.2)	1m 24s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	1m 21s
specs (macos, 3.1.4)	1m 23s
specs (macos, 3.2.2)	2m 2s
specs (macos, 3.3.0)	1m 7s
specs (windows, 3.0.6)	4m 31s
specs (windows, 3.1.4)	4m 50s
specs (windows, 3.2.2)	4m 46s
specs (windows, 3.3.0)	4m 29s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 22s
	32m 4s

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