sheep: fix hang when IO NIC is down only

If IO NIC is down but sheep alive, epoch isn't incremented, so we can't retry
poll for ever.

This problem can be demonstrated by 050.

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