destroy_async
Description:
Destroy a PKCS#11 object, deleting it from storage or the session.
This call will return immediately and complete asynchronously.
Parameters:
| this | 
           The object to destroy.  | 
      
| cancellable | 
           Optional cancellable object, or   | 
      
| callback | 
           Callback which is called when operation completes.  | 
      
| user_data | 
           Data to pass to the callback.  |