Nodes, Peers and Topology methods for node

Topology accepts an argument to define the depth of the topology
requested from the network. proto definitions have been modified
accordingly, too.

network/network.go

@@ -46,6 +46,8 @@ type Network interface {
 	Connect() error
 	// Nodes returns list of network nodes
 	Nodes() []Node
+	// Topology returns a list of all reachable nodes up to depth
+	Topology(depth uint) []Node
 	// Close stops the tunnel and resolving
 	Close() error
 	// Client is micro client