remove static string compressor from QueryCompletionIndex

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

@@ -6,6 +6,7 @@ package org.lucares.pdb.api;
  * 'Sam' is the value.
  */
 public class Tag implements Comparable<Tag> {
+
     private final int field;
 
     private final int value;
@@ -42,6 +43,10 @@ public class Tag implements Comparable<Tag> {
         return value;
     }
 
+    public Tag unsetValue() {
+        return new Tag(field, -1);
+    }
+
     @Override
     public String toString() {
         return field + "=" + value;