jc_utils: fix the broken stage_info_opt function.

This function should return a stage if the stage is found.  However, it was
reusing a bounded argument of a different type.
jc_utils: add the no_tasks_running function.

This function checks to see if there is any tasks running in the current
stage.
Start the tasks concurrently.
comments.
job_coordinator: serialize the termination of different stages.
job_coordinator: combine the stages.