rgw: copy_obj_data() uses atomic processor

Fixes: #9089
copy_obj_data was not using the current object write infrastructure,
which means that the end objects weren't striped.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>

Conflicts:
	src/rgw/rgw_rados.cc
rgw: pass set_mtime to copy_obj_data()

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>