[TDF] When no data-source is present, spawn two tasks per worker
[TDF] When no data-source is present, spawn two tasks per worker

This is a simple fix to reduce imbalance in multi-thread event-loops
with no data-source: previously we spawned one task per worker thread.

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