SILC Async Operation API is an interface that can be used to control
 asynchronous operations.  All functions that take callback as argument
 should return SilcAsyncOperation context.  That context then can be
 used to control, such as, abort the asynchronous operation.  Using
 SILC Async Operation API, asynchronous functions can be controlled
 and aborted safely.

 The SILC Async Operation API is divided in two levels; the underlaying
 operation level that implements the asynchronous operation, and the
 upper layer that can control the asynchronous operation.  The operation
 layer must guarantee that if the upper layer aborts the asynchronous
 operation, no callback function will be called back to the upper layer.
 This must be remembered when implementing the operation layer.