Make sure submissions run through multi-task phases in the same order

If a phase has more than one task, the order submissions get sent to them should be the same for every submission.

Current behavior:

parent sub n
  child n:   task 1
  child n+1: task 3
  child n+2: task 2

parent sub n+1
  child n:   task 2
  child n+1: task 1
  child n+2: task 3

should be:

parent sub n
  child n:   task 1
  child n+1: task 3
  child n+2: task 2

parent sub n+1
  child n:   task 1
  child n+1: task 3
  child n+2: task 2

so even if the tasks order isn't configurable, the nth child will always be run against the same datasets.