[sheepdog] add dual nic support

[Liu Yuan]

This patch set add dual nic support. Usage:

$ sheep -i ip{,port} ... # this add a dedicated io nic

We use redundant nic primarily for IO requests and we can also fallback on the
non-io connection when IO nic is down.

We support cluster with heterogeneous NIC configuration but the main purpose is to
allow separation of IO requests and cluster membership heartbeat messages to get
better reliability and scalability if secondary NIC is provided on all the nodes.

Yuan