typedef SilcBool (*SilcAsyncOperationPause)(SilcAsyncOperation op,
                                                SilcBool pause_operation,
                                                void *context);

    This callback is used to halt an operation, if upper layer calls the
    silc_async_halt function, or to resume an operation if upper layer
    calls the silc_async_resume, after it has earlier halted the operation.
    If this callback is implemented it is guaranteed that the asynchronous
    operation is not progressed when it is halted.  If the `pause_operation'
    is TRUE the operation is halted.  If it is FALSE, then the operation
    resumes its execution.  This function returns TRUE if the operation
    was (or is going to be) halted or resumed, and FALSE on error.