Changed router interface. Added table watcher. Advertise routes

* Changed router interface to return Advertisement channel
* Added default gateway route to the routing table if supplied
* Watch table for updates and advertise to the network
* We hash the routes on 3-tuple (Destination, Gateway, Network)

network/router/router.go

@@ -1,6 +1,11 @@
 // Package router provides a network routing control plane
 package router
 
+var (
+	// DefaultRouter is default network router
+	DefaultRouter = NewRouter()
+)
+
 // Router is an interface for a routing control plane
 type Router interface {
 	// Init initializes the router with options
@@ -15,21 +20,27 @@ type Router interface {
 	Address() string
 	// Network returns the network address of the router
 	Network() string
-	// Advertise starts advertising the routes to the network
-	Advertise() error
+	// Advertise starts advertising routes to the network
+	Advertise() (<-chan *Advertisement, error)
+	// Update updates the routing table
+	Update(*Advertisement) error
 	// Stop stops the router
 	Stop() error
 	// String returns debug info
 	String() string
 }
 
+// Advertisement is sent by the router to the network
+type Advertisement struct {
+	// ID is the source router ID
+	ID string
+	// Event defines advertisement even
+	Event *Event
+}
+
 // Option used by the router
 type Option func(*Options)
 
-var (
-	DefaultRouter = NewRouter()
-)
-
 // NewRouter creates new Router and returns it
 func NewRouter(opts ...Option) Router {
 	return newRouter(opts...)