Shift marker persistence off of server thread (lag reduction)

src/main/java/org/dynmap/bukkit/DynmapPlugin.java

@@ -256,6 +256,11 @@ public class DynmapPlugin extends JavaPlugin implements DynmapAPI {
                 return true;
             return false;
         }
+        @Override
+        public boolean isServerThread() {
+            return Bukkit.getServer().isPrimaryThread();
+        }
+
         @Override
         public String stripChatColor(String s) {
             return ChatColor.stripColor(s);