Persistence mode

Persistence mode is defined in an enum type diet_persistence_mode_t

mode	Description
DIET_VOLATILE	not stored
DIET_PERSISTENT_RETURN	stored on server, movable and copy back to client
DIET_PERSISTENT	stored on server and movable
DIET_STICKY	stored and non movable
DIET_STICKY_RETURN	stored, non movable and copy back to client

NB:

DIET_STICKY_RETURN is not implemented yet.