src/test/osd: restructure Object/RadosModel in prep for append

Attribute handling no longer has special support in ContentsGenerator.
The most recent operation information is now stored in a special
attr rather than at the beginning of the object.  ObjectDesc layers
include their own ContentsGenerators to allow more flexibility.

Also, writes truncate to the new object size rather than simply
causing reads to stop at that object size.

Signed-off-by: Samuel Just <sam.just@inktank.com>