sheep: fix automatic stale object purging

If sheep process have differert disk size,after recovery. ./stale directory
still have objects.because in the cluster_recovery_completion function,
the memcmp function parameter vnode_info->nodes and recovereds will have
different nr_vnodes number after recovery completion.so i just compare
node_id to fix this problem.

Signed-off-by: wangfan <wangfan1985@gmail.com>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
tests/054: test automatic stale object purging

test whether .stale directory has a object file.
if use -md option,it can also check.

Signed-off-by: wangfan <wangfan1985@gmail.com>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>