Add Node's optional attribute for external communications

[carpawell]

Describe an (optional) Node's attribute that declares a preferred way for external communications. It should contain a list of multiaddresses.

According to current behviour, nodes get peer address from the network map entries. Client software either does the same or uses endpoint set in config.

If a node admin wants to use separate interfaces for serving clients and for p2p communication with other storage nodes, he sets one more listening address in node config and uses it as connection endpoints in clients (protocol gateways, applications, etc).

Adding well known attributes for additional endpoint could simplify this process. Client libraries used in, for example, protocol gateways could correctly discover and use proper endpoints without mixing client traffic with internal replication traffic.