[TDF] Fix one-time execution of callbacks
[TDF] Fix one-time execution of callbacks

When there are less tasks than workers slot number 0 is never used:
we use slot numbers from the highest down. So if we want to execute
a callback just once for all the slots the safe thing to do is
executing it in the higher slot number.

zzxuanyuan/root,olifre/root,karies/root,zzxuanyuan/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,karies/root,zzxuanyuan/root,olifre/root,olifre/root,karies/root,zzxuanyuan/root,olifre/root,zzxuanyuan/root,zzxuanyuan/root,olifre/root,karies/root,karies/root,olifre/root,zzxuanyuan/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,zzxuanyuan/root,karies/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,zzxuanyuan/root,olifre/root,zzxuanyuan/root,root-mirror/root,karies/root,root-mirror/root,zzxuanyuan/root,root-mirror/root,zzxuanyuan/root,root-mirror/root