Allow a single worker process to manage multiple children

Rather than running N separate processes on a worker node to
process N simultaneous jobs, use a single parent worker process
which will poll for jobs and launch child process as needed up
to a maximum defined by conf value max_child_processes.

Includes a refactoring to clean up and separate the
single process and parent process worker code to eliminate
a lot of conditional code depending on configuration.

Also updated logging to be more consistent and to record worker
id, parent pid, and child pid (where applicable) to help with
tracing.
Well, the _check_children call doesn't actually wait. It cycles thru the list and just checks each child's status so the parent doesn't block during that time. But looking over this bit I think it probably can (and should) be refactored a little so it isn't as repetitious. (Still wish python had a do-while!)