LP#1768947 Offline xact presence is cached; show date

LP#1768947 Offline xact presence is cached; show date

Maintain an entry to the object date cache table indicating the time of
the most recent offline transaction entry.  This data is used on the
login page to determine if offline transactions exist, so the staff
logging in can be notified.  We do this in lieu of checking the offline
transaction table, since that table only exists in the offline UI.

As a bonus, since we know the last transaction add time, display this
information in the login page offline xact alert panel.

Signed-off-by: Bill Erickson <berickxx@gmail.com>