val annotation: KClass<out Annotation>
Indicate the 'async' annotation type to be detected at either class or method level.
By default, both Spring's @Async annotation and the EJB 3.1 @javax.ejb.Asynchronous annotation will be detected.
This attribute exists so that developers can provide their own custom annotation type to indicate that a method (or all methods of a given class) should be invoked asynchronously.