use listing.csv instead of iterating through all folders

The hope is, that it is faster to read a single file instead of listing
hundreds of folders.

performanceDb/src/test/java/org/lucares/performance/db/PerformanceDbTest.java

@@ -113,7 +113,7 @@ public class PerformanceDbTest {
 
 			final List<Path> filesInStorage = FileUtils.listRecursively(DataStore.storageDirectory(dataDirectory));
 
-			Assert.assertEquals(filesInStorage.size(), 1);
+			Assert.assertEquals(filesInStorage.size(), 2, "the created file and the listing.csv");
 
 			final Path tagSpecificFile = filesInStorage.get(0);