sheep: make failed_nodes and delayed_nodes static variables

This also guards them with the main_thread macro because they are not
thread-safe.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep/group: allow sending SD_OP_GET_EPOCH to delayed nodes

We cannot check whether the node is a delayed one or not in
epoch_log_read_remote() because we cannot access delayed node list in
the worker thread.  It is not a problem to send SD_OP_GET_EPOCH to
delayed nodes, so let's simply allow it.

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