cache: Introduce StringCache, switch to unordered_map, default to memory cache if fs cache dir not given

qswiki.cpp

@@ -72,7 +72,7 @@ std::unique_ptr<ICache> createCache(const ConfigVariableResolver &resolver)
 	std::string path = resolver.getConfig("cache_fs_dir");
 	if(path == "")
 	{
-		return std::make_unique<NoCache>(path);
+		return std::make_unique<StringCache>();
 	}
 	return std::make_unique<FsCache>(path);
 }