rgw: reduce calls to rgw_obj.set_obj()

default manifest iterator construction did unneeded init of rgw_obj.
Also add a smarter copy constructor to RGWObjState that does not copy
manifest if not needed.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1863 from ceph/wip-rgw-manifest-iter

rgw: reduce calls to rgw_obj.set_obj()

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>