typedef void (*SilcFSMThreadDestructor)(SilcFSMThread thread,
                                            void *thread_context,
                                            void *destructor_context);

    The destructor callback that was set in silc_fsm_thread_alloc or in
    silc_fsm_thread_init function.  It will be called when a state function
    returns SILC_FSM_FINISH.  This function will be called through the
    scheduler; it will not be called immediately after the state function
    returns SILC_FSM_FINISH, but will be called later.  The `thread' can
    be freed in this function.

    Even if the `thread' was executed in real system thread, this callback
    is always received in the main machine thread, not in the created
    thread.