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.

pdb-utils/src/main/java/org/lucares/utils/Preconditions.java

@@ -58,4 +58,10 @@ public class Preconditions {
 		}
 	}
 
+	public static void checkNull(final Object actual, final String message, final Object... args) {
+		if (actual != null) {
+			throw new IllegalStateException(MessageFormat.format(message, args));
+		}
+	}
+
 }