Remember cache input data (#133)

Make init_cache and set_cache no-ops if fed the same data as before

src/virtual_machine.hpp

@@ -65,6 +65,8 @@ protected:
 		randomx_dataset* datasetPtr;
 	};
 	uint64_t datasetOffset;
+public:
+	std::string cacheKey;
 };
 
 namespace randomx {
@@ -80,4 +82,4 @@ namespace randomx {
 		void generateProgram(void* seed);
 	};
 
-}
+}