prepare more efficient query completion

adding an index that answers the question
given a query "a=b and c=", what are possible values
for c.

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

@@ -8,6 +8,7 @@ import java.util.TreeSet;
 import java.util.function.BiConsumer;
 import java.util.function.Function;
 
+import org.lucares.collections.IntList;
 import org.lucares.collections.LongList;
 import org.lucares.utils.byteencoder.VariableByteEncoder;
 
@@ -156,6 +157,14 @@ public class Tags implements Comparable<Tags> {
 		return result;
 	}
 
+	public IntList getKeysAsInt() {
+		final IntList result = new IntList();
+		for (final Tag tag : tags) {
+			result.add(tag.getKey());
+		}
+		return result;
+	}
+
 	public List<Tag> toTags() {
 		return Collections.unmodifiableList(tags);
 	}