Simplify the function that executes a WorkflowTask list

[tcompa]

In view of #261, we may want to refactor the recursive_task_submission function. This currently has a recursive structure, where N tasks are split into (N-1) "dependency" tasks, plus the last one. And then the function is called recursively on the (N-2)+1 tasks, then on the (N-3)+1 tasks, and so on, until it receives a list of a single task and executes it.

We should check whether it's worth to make it flat.

Also: this function's extensive use of futures is partly useless, since we always wait for the future result before moving on to the next task.

We should check whether this can be simplified without loosing any functionality, since this will make work on #261 more transparent.

[tcompa]

Closed with #780