carbonara: serialize time period in seconds
carbonara: serialize time period in seconds

It's better than using custom pandas rule codes.
Our code can still read the old format anyway.

Change-Id: Iae9b3485a32dc98b5bef6128ac6386529aa9705f

gnocchixyz/gnocchi,sileht/gnocchi,leandroreox/gnocchi,leandroreox/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi