#937 Add option for AuthMe to run in sync

- Create BukkitService#runTaskOptionallyAsync and BukkitService#scheduleSyncTaskFromOptionallyAsyncTask whose behavior depends on a new setting
- Use the new methods where applicable
- Declare events async or sync depending on the new setting

src/main/java/fr/xephi/authme/events/ProtectInventoryEvent.java

@@ -22,9 +22,10 @@ public class ProtectInventoryEvent extends CustomEvent implements Cancellable {
      * Constructor.
      *
      * @param player The player
+     * @param isAsync True if the event is async, false otherwise
      */
-    public ProtectInventoryEvent(Player player) {
-        super(true);
+    public ProtectInventoryEvent(Player player, boolean isAsync) {
+        super(isAsync);
         this.player = player;
         this.storedInventory = player.getInventory().getContents();
         this.storedArmor = player.getInventory().getArmorContents();