Support asynchronous events; Addresses BUKKIT-1212

By: Wesley Wolfe <weswolf@aol.com>

paper-api/src/main/java/org/bukkit/Server.java

@@ -639,4 +639,9 @@ public interface Server extends PluginMessageRecipient {
      * @returns The water animal spawn limit
      */
     int getWaterAnimalSpawnLimit();
+
+    /**
+     * Returns true if the current {@link Thread} is the server's primary thread
+     */
+    boolean isPrimaryThread();
 }