#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/test/java/fr/xephi/authme/TestHelper.java

@@ -87,6 +87,20 @@ public final class TestHelper {
         runnable.run();
     }
 
+    /**
+     * Execute a {@link Runnable} passed to a mock's {@link BukkitService#runTaskOptionallyAsync} method.
+     * Note that calling this method expects that there be a runnable sent to the method and will fail
+     * otherwise.
+     *
+     * @param service The mock service
+     */
+    public static void runOptionallyAsyncTask(BukkitService service) {
+        ArgumentCaptor<Runnable> captor = ArgumentCaptor.forClass(Runnable.class);
+        verify(service).runTaskOptionallyAsync(captor.capture());
+        Runnable runnable = captor.getValue();
+        runnable.run();
+    }
+
     /**
      * Execute a {@link Runnable} passed to a mock's {@link BukkitService#scheduleSyncDelayedTask(Runnable)}
      * method. Note that calling this method expects that there be a runnable sent to the method and will fail
@@ -115,6 +129,20 @@ public final class TestHelper {
         runnable.run();
     }
 
+    /**
+     * Execute a {@link Runnable} passed to a mock's {@link BukkitService#scheduleSyncTaskFromOptionallyAsyncTask}
+     * method. Note that calling this method expects that there be a runnable sent to the method and will fail
+     * otherwise.
+     *
+     * @param service The mock service
+     */
+    public static void runSyncTaskFromOptionallyAsyncTask(BukkitService service) {
+        ArgumentCaptor<Runnable> captor = ArgumentCaptor.forClass(Runnable.class);
+        verify(service).scheduleSyncTaskFromOptionallyAsyncTask(captor.capture());
+        Runnable runnable = captor.getValue();
+        runnable.run();
+    }
+
     /**
      * Assign the necessary fields on ConsoleLogger with mocks.
      *