Make CCoinsViewCache's copy constructor private
Make CCoinsViewCache's copy constructor private

It is easily confused with CCoinsViewCache(CCoinsView*), which creates a sub-cache, but instead of creating a sub-cache, the copy constructor would copy the original and use that original's base, defeating the intended isolation.

Coming from upstream@228d2385254ee136ef64d229a414fdd7cefea9a0

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