Check performance of tset artifactvalues cache

[cjhopman]

There's an interesting issue in bazel here: https://github.com/bazelbuild/bazel/discussions/21378. There's a repro for a bad perf case of their merkle tree cache here: https://github.com/DavidANeil/repro-bazel-nested-set/tree/master. That cache seems like it likely does something very similar to how we cache the ActionSharedDirectory for tset nodes, the repro might be a good example for us to check performance on.