osd: move object_info_t, exists, size into ObjectState

osd: move object_info_t, exists, size into ObjectState

The main goal is to capture everything needed by the op context
(the object_info_t, exists, size) in something that doesn't include all
of ObjectContext (such as teh current access mode, num writers, and other
things that only the primary needs) so that the sub_op_modify setup is
as simple as possible.

This kills the annoying exists, size args in all the helpers.