jobpack: Add the concurrent flag to the pipeline stages.

Now whether a stage can run concurrently or not can be specified by the client
side.  This functionality is still hard-coded in the python client.
jc_utils: modify new_task_permitted_for_stage to use concurrency info.

With the new approach, a task can run if either all of the dependencies of the
stage have finished or the task belongs to a concurrent stage.