libutil: Use ThreadPool in computeClosure to parallelize promise resolution #21796

Job	Run time
pre-commit checks	1m 18s
eval	48s
Check presence of secrets	3s
aggregate basic checks	3s
tests on ubuntu	42m 51s
tests on macos	20m 47s
tests on ubuntu (with sanitizers / coverage)	32m 53s
installer test on macos	1m 29s
installer test on ubuntu	39s
installer test on macos (experimental)	2m 39s
flake_regressions	4m 31s
installer test on ubuntu (experimental)	1m 20s
docker_push_image	0s
profile_build	0s
	1h 49m 21s

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