leverage the cached pdbwriters

this increased performance from 500 entries per second to 4000.

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

@@ -16,6 +16,8 @@ public class Tags {
 
 	private final Map<String, Tag> tags;
 
+	private int cachedHash = 0;
+
 	private Tags() {
 		super();
 		tags = Collections.emptyMap();
@@ -87,10 +89,17 @@ public class Tags {
 
 	@Override
 	public int hashCode() {
-		final int prime = 31;
-		int result = 1;
-		result = prime * result + ((tags == null) ? 0 : tags.hashCode());
-		return result;
+
+		if (cachedHash != 0) {
+			return cachedHash;
+		} else {
+
+			final int prime = 31;
+			int result = 1;
+			result = prime * result + ((tags == null) ? 0 : tags.hashCode());
+			cachedHash = result;
+			return result;
+		}
 	}
 
 	@Override