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-api/src/main/java/org/lucares/pdb/api/Tags.java

@@ -39,6 +39,10 @@ public class Tags {
 		filenameBytes = EMPTY_BYTES;
 	}
 
+	public Tags(final byte[] filenameBytes) {
+		this(new String(filenameBytes, StandardCharsets.UTF_8));
+	}
+
 	public Tags(final String serializedTags) {
 		// serialized tags look like this: 0-1_2-1M_H-28_4-5$1.pdb
 		// there can be several files for the same set of tags, in which case the number
@@ -85,6 +89,10 @@ public class Tags {
 		return new String(this.filenameBytes, StandardCharsets.UTF_8);
 	}
 
+	public byte[] getFilenameBytes() {
+		return filenameBytes;
+	}
+
 	public String getValue(final String key) {
 
 		final Set<Tag> tags = toTags();
@@ -96,7 +104,7 @@ public class Tags {
 		return null;
 	}
 
-	private SortedSet<Tag> toTags() {
+	public SortedSet<Tag> toTags() {
 		final SortedSet<Tag> result = new TreeSet<>(TagByKeyComparator.INSTANCE);
 		final String filename = new String(this.filenameBytes, StandardCharsets.UTF_8);
 		final Matcher matcher = EXTRACT_TAGS_PATTERN.matcher(filename);