cluster/zookeeper: add log information for zk auto-recoonect

Reported-by: Valerio Pachera <sirio81@gmail.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog/farm: multi-thread cluster snapshot save process

There is no reason we can't multi-thread it. Let's go multi-threading and enjoy
boost.

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