Remember cache input data (#133)

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

src/dataset.hpp

@@ -50,6 +50,7 @@ struct randomx_cache {
 	randomx::DatasetInitFunc* datasetInit;
 	randomx::SuperscalarProgram programs[RANDOMX_CACHE_ACCESSES];
 	std::vector<uint64_t> reciprocalCache;
+	std::string cacheKey;
 
 	bool isInitialized() {
 		return programs[0].getSize() != 0;