Use SipHash-2-4 for CCoinsCache index

This is ~1.7x slower than the Lookup3-of-Xor-with-salt construct we were
using before, but it is a primitive designed for exactly this.

src/uint256.h

@@ -140,11 +140,6 @@ public:
     {
         return ReadLE64(data);
     }
-
-    /** A more secure, salted hash function.
-     * @note This hash is not stable between little and big endian.
-     */
-    uint64_t GetHash(const uint256& salt) const;
 };
 
 /* uint256 from const char *.