wait for threads to end via tbbs observer

we currently do not wait for all the threads to end, which may result in undefined behaviour at the end of the eventloop. This aims to fix this issue. I also tried task_groups with task_arenas, but since tbb

Also, I tried to remove Gaudi::Hive::setCurrentContextEvt(-1) and saw no difference in anything. @sponce do you by any chance know why this is there ( i copied this from the eventloopmgr ). If not and we see no difference in the nightlies, we could remove it?