Comment 1 for bug 1748924

Branch for consideration at:
http://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/dbwells/lp1748924_redo_billing_timestamps

working/dbwells/lp1748924_redo_billing_timestamps

This is a major rethink of some billing timestamp work done locally a few years back (see LP#1422379), and now that it is done, I think we may still end up reimplementing a COALESCE(period_start, create_date) single value in the long run, but we'll see how that plays out.

This branch attempts to make the minimum changes to allow for this DB enhancement. Should this be accepted, I have several other bug fixes which build on the availability of these new values.