osd: Add a new object_copy_data_t, and use it in the OSD/Objecter

osd: Add a new object_copy_data_t, and use it in the OSD/Objecter

Right now this is very primitive, but we're about to extend it to
deal with request versioning appropriately, and adding in some
extra fields.
Sadly we are doing a little extra copying in the Objecter as a result, but
too bad -- being able to do updates will be worth it.

Signed-off-by: Greg Farnum <greg@inktank.com>