LP#1086458: convert last-print information hash to JSON string before caching

LP#1086458: convert last-print information hash to JSON string before caching

By serializing the printing message and context information to a
JSON string before caching it, we avoid inadvertantly dragging in
things like Javascript execution contexts and possibly references
to objects created by the checkout interface.  This was contributing
to the staff client memory leaks observed during receipt printing.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Dan Wells <dbw2@calvin.edu>