add debug warning when cleanup thread is not finished

[dereyck]

Introduced a debug warning in the scheduler to log when the cleanup thread
has not finished cleaning up all threads marked for destruction before it
is descheduled. This ensures that any unfinished cleanup work is flagged,
helping to identify potential issues with thread management or incomplete
cleanup processes in the system.

[dgruss]

this patch would slow down things, potentially exacerbating the problem.
i would rather add a counter in the scheduler that is atomically increased/decreased and print a warning in the scheduler when the last thread was the cleanupthread and the counter is > 5 or so...