rewrite query completion

The old implementation searched for all possible values and then
executed each query to see what matches.
The new implementation uses several indices to find only
the matching values.

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

@@ -11,8 +11,8 @@ public class Tag implements Comparable<Tag> {
 	}
 
 	public Tag(final String key, final String value) {
-		this.key = Tags.STRING_COMPRESSOR.put(key);
-		this.value = Tags.STRING_COMPRESSOR.put(value);
+		this.key = key != null ? Tags.STRING_COMPRESSOR.put(key) : -1;
+		this.value = value != null ? Tags.STRING_COMPRESSOR.put(value) : -1;
 	}
 
 	@Override

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

@@ -72,6 +72,13 @@ public class Tags implements Comparable<Tags> {
 		return result;
 	}
 
+	public static Tags create(final String key1, final String value1, final String key2, final String value2,
+			final String key3, final String value3, final String key4, final String value4) {
+		final Tags result = TagsBuilder.create().add(key1, value1).add(key2, value2).add(key3, value3).add(key4, value4)
+				.build();
+		return result;
+	}
+
 	public static Tags fromBytes(final byte[] bytes) {
 		final List<Tag> result = new ArrayList<>();
 
@@ -188,7 +195,7 @@ public class Tags implements Comparable<Tags> {
 
 	@Override
 	public String toString() {
-		return "Tags [tags=" + tags + "]";
+		return String.valueOf(tags);
 	}
 
 	@Override