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/proto/network.proto

@@ -8,6 +8,7 @@ import "github.com/micro/go-micro/router/proto/router.proto";
 service Network {
         rpc ListNodes(ListRequest) returns (ListResponse) {};
         rpc ListPeers(PeerRequest) returns (PeerResponse) {};
+        rpc Topology(TopologyRequest) returns (TopologyResponse) {};
         rpc ListRoutes(go.micro.router.Request) returns (go.micro.router.ListResponse) {};
 }
 
@@ -28,15 +29,20 @@ message PeerRequest {
 
 // PeerResponse is returned by ListPeers
 message PeerResponse {
-        Peers peers = 1;
+        repeated Node peers = 1;
 }
 
-// Peers are node peers
-message Peers {
-        // network node
-        Node node = 1;
-        // node peers
-        repeated Node peers = 2;
+// TopologyRequest list node topology
+message TopologyRequest {
+        // node id
+        string id = 1;
+        // topology depth
+        uint64 depth = 2;
+}
+
+// TopologyResponse is returned by Topology
+message TopologyResponse {
+        Topology topology = 1;
 }
 
 // Node is network node
@@ -47,6 +53,14 @@ message Node {
         string address = 2;
 }
 
+// Topology is used to nnounce node neighbourhood
+message Topology {
+        // network node
+        Node node = 1;
+        // neighbours
+        repeated Node nodes = 2;
+}
+
 // Connect is sent when the node connects to the network
 message Connect {
         // network mode