use PersistentMap in DataStore

Replaces the use of in-memory data structures with the PersistentMap.
This is the crucial step in reducing memory usage for both persistent
storage and main memory.

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

@@ -15,7 +15,6 @@ import java.util.Random;
 import java.util.UUID;
 import java.util.concurrent.atomic.AtomicInteger;
 
-import org.lucares.pdb.map.PersistentMap.Visitor;
 import org.lucares.utils.file.FileUtils;
 import org.testng.Assert;
 import org.testng.annotations.AfterMethod;