ENH Improve & simplify timed_path
ENH Improve & simplify timed_path

The most user visible result is that this removes the ugly
``jug.utils._return_first`` task from the task list. It was an
implementation detail that shouldn't have been visible.

This will also trigger recomputation of any previous results,
unfortunately.

unode/jug,unode/jug,luispedro/jug,luispedro/jug