SPIGOT-443: Synchronize access to HANDLED_TAGS as Spigot uses it async

src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java

@@ -691,6 +691,7 @@ class CraftMetaItem implements ItemMeta, Repairable {
     }
 
     public static Set<String> getHandledTags() {
+        synchronized (HANDLED_TAGS) {
             if (HANDLED_TAGS.isEmpty()) {
                 HANDLED_TAGS.addAll(Arrays.asList(
                     DISPLAY.NBT,
@@ -714,3 +715,4 @@ class CraftMetaItem implements ItemMeta, Repairable {
             return HANDLED_TAGS;
         }
     }
+}