Network: support partial meshes

[reinkrul]

• Reduces overload of chatter for broadcast messages (e.g. gossips)
• Improves scalability: allows much larger networks (e.g. cross domain)
• Improves stability: when private TX payload can be routed over other nodes, peers don't need a 1:1 connection to nodes to exchange private information (to be checked: is the legally allowed?)

[woutslakhorst]

Some more explanation required