tag: optimize tag_dup(), copy item references

Don't call tag_pool_get_item() for duplicating tags, just increase the
item's reference counter instead.

src/tag_pool.h

@@ -26,6 +26,8 @@ struct tag_item;
 struct tag_item *tag_pool_get_item(enum tag_type type,
 				   const char *value, int length);
 
+struct tag_item *tag_pool_dup_item(struct tag_item *item);
+
 void tag_pool_put_item(struct tag_item *item);
 
 #endif