zookeeper: remove unneccessary zk operation

Current implementation of zk_handle_unblock() sends an NOTIFY event to
all sheep. All sheep who receive NOTIFY event will call
sd_notify_handler() to do unblock.
However, this is unncessary, we can call sd_notify_handler() directly
in zk_handle_unblock().
And this how corosync driver handle unblock.

Signed-off-by: Kai Zhang <kyle@zelin.io>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: handle error of notify_vdi_add()

Signed-off-by: Kai Zhang <kyle@zelin.io>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>