BSFile uses a wrapper for DiskBlock to add BSFile specific stuff

This keeps the DiskBlock class clean, so that it can be used
for PersistentMap.

block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java

@@ -6,6 +6,7 @@ import java.nio.file.Path;
 
 import org.lucares.pdb.diskstorage.DiskStorage;
 import org.lucares.utils.file.FileUtils;
+import org.testng.Assert;
 import org.testng.annotations.AfterMethod;
 import org.testng.annotations.BeforeMethod;
 import org.testng.annotations.Test;
@@ -31,8 +32,11 @@ public class PersistentMapTest {
 		try (final DiskStorage ds = new DiskStorage(file)) {
 			final PersistentMap map = new PersistentMap(ds);
 
-			map.put("key1", "value1");
-			map.getAsString("key1");
+			final String value1 = "value1";
+			map.put("key1", value1);
+			final String actualValue1 = map.getAsString("key1");
+
+			Assert.assertEquals(actualValue1, value1);
 		}
 	}
 }