ObjectStore: use ghobject_t for setattrs

Signed-off-by: Samuel Just <sam.just@inktank.com>
src/osd/: add ECBackend stubs

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/ECTransaction: add

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