- context::terminated_is_linked() might be missleading because this
state is transient
- if the context is pushed to the terminated-queue,
context::terminated_is_linked() returns true, if the context is
removed from terminated-queue context::terminated_is_linked() returns
false
- new flag flag_terminated intoduced
- flag_terminated will be set in context::set_terminated_(), which will
be called for worker context' in the lambda
- context::join() adds active-context to wait-queue only
if joined context is not terminated
- we can not use terminated_is_linked() because the context
might already be removed from scheudler's termianted-queue
