sheep/http: only read name of kv_onode when iterating

In object_iterater() it read whole length of SD_DATA_OBJ_SIZE data for
onode which is not necessary and will hurt performance badly.
We only need to read out the 'name' of onode.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>