[GNC] gncJobNextID in gncJob.c not in gncJob.h

[Robert Simmons]

> Accounting systems primarily perform the same operation on large numbers
of data elements.

I wish I had time to devote to this. I use graph databases in many of the
other systems that I develop. This would be a great pivot point on which to
create a new open source project and see which paradigm really is better.
But really I need to get my data organized and then handed off to an
accountant (so I can get back to working on my real work).