sheep: clean up node comparison

Use node_is_local() to check that the node is local, use node_eq() to
check two nodes are the same one, and remove unnecessary function
is_myself().

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
recovery: check whether the object is already scheduled at first

We can do the check before finding the target nodes.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>