jobpack: Add the concurrent flag to the pipeline stages.

The stage() type has been modified to include a third field showing whether the
stage should run concurrently or sequentially.  By default, all of the stages
should run sequentially but it will be changed into a configurable option so
that it can be turned on for specific stages in the pipeline model.
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.
pipeline worker: let the users specify concurrent stages.