sheep: fix explanation of SD_RES_OLD_NODE_VER and SD_RES_NEW_NODE_VER

Sheep returns SD_RES_OLD_NODE_VER when the sent request contains an
older epoch, and SD_RES_NEW_NODE_VER when it contains an newer one.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
zookeeper: return the found path in zk_find_seq_node

This is necessary for the zookeeper driver to get the queue position
on the first push.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: gateway cannot cleanup stale objects

sd_store can be NULL if this node is a gateway, so we need the NULL
check before calling sd_store->cleanup.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
[tmp]

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>