sheep/http: rework kv_find_object()

We duplicately call vdi_lookup twice for every object operation, which isn't
necessary. This patch reduce it to a single call of vdi_lookup.

Also rename kv_list_{objects, buckets} as kv_iterate_{object, bucket}

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