test/osd/Object: create encoders for ContDesc

Signed-off-by: Samuel Just <sam.just@inktank.com>
src/test/osd: remove tmap test

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

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