sheep/cluster: remove ephemeral node when zookeeper master leaves

The zookeeper node /sheepdog/master/000000xxxx is ephemeral, but when
the sheep process which is master in the zookeeper cluster leaves,
zookeeper will not remove the ephemeral node immediately. If the sheep
restarts at once, the sheepdog cluster will be corrupted.

This patch fixes it by trying removing the ephemeral node when
sheep leaves the cluster.

Typo for MASTER_ZNODE is also fixed in the patch.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep/request: fix fd leak

This patch fixes fd leak in sheep process such as below in lsof command.

sheep 15276 liangry 95u sock 0,7 0t0 465109 can't identify protocol
sheep 15276 liangry 96u sock 0,7 0t0 464282 can't identify protocol
sheep 15276 liangry 97u sock 0,7 0t0 464284 can't identify protocol

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>