Merge pull request #2905 from stan-dev/feature/threadsafe-matrixcl #3660

Job	Run time
fwd tests and non-fun mix tests	41m 48s
mix/fun tests 2	1h 57m 26s
mix/fun tests 1	2h 5m 43s
prim and rev tests	2h 17m 52s
	7h 2m 49s

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