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.

src/coins.h

@@ -441,6 +441,11 @@ public:
 private:
     CCoinsMap::iterator FetchCoins(const uint256 &txid);
     CCoinsMap::const_iterator FetchCoins(const uint256 &txid) const;
+
+    /**
+     * By making the copy constructor private, we prevent accidentally using it when one intends to create a cache on top of a base cache.
+     */
+    CCoinsViewCache(const CCoinsViewCache &);
 };
 
 #endif // BITCOIN_COINS_H