Wait at the end of `@parallel` for loop w/o reduce

As-is, it seems like parallel for without reduction spawns all jobs and never waits for them. I think it should have a barrier at the end of the for, otherwise a program which contains only this

```julia
@parallel for l in lines
  workon_and_save_result_somewhere(l)
end
```

will quit immediately after having spawned all workers, i.e. it's useless. I'm not sure what the rationale of the current behavior is, or if I'm missing something obvious?