sheep: add debug info in zookeeper.c

Set sheep node information in zookeeper files so we can debug it easier when
there are same problmes happens in cluster.

And, add come sd_debug() in zookeeper.c

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: avoid dead loop when zookeeper session is expired

When zookeeper session is expired, zk_get_least_seq() will return
ZNONODE, but we have not check the return value of zk_get_least_seq()
so the routine will fall in dead loop.

To avoid it, we add check for return value of zk_get_least_seq() and
jump to recreate sequential-node if find out the ZNONODE event.

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