typedef void (*SilcAsyncOperationAbort)(SilcAsyncOperation op,
                                            void *context);

    This callback is called when upper layer calls the silc_async_abort,
    and is used to actually perform the abortion of the asynchronous
    operation.  The silc_async_free must not be called in this function.

    This callback type can also be provided to silc_async_abort function
    by the upper layer, if it wants that callback is called to the upper
    layer when aborting the operation.