LP#1748924 Enhanced Billing Timestamp Support

LP#1748924 Enhanced Billing Timestamp Support

As discussed at Hack-a-way 2016, rather than continue to try to cram
multiple meanings into one timestamp, let's create a complete set of
all the useful timestamps for a typical billing.

In this new config, every billing will have a 'create_date', then most
(overdues) will also describe when they start and end.

billing_ts is now deprecated, but will continue to exist for backwards
compatibility.  It will be managed by trigger to approximate its
current definition; equal to 'period_end' for overdues, equal to
'create_date' for other billings.

Signed-off-by: Dan Wells <dbw2@calvin.edu>