graph: export addZombieEdge and rename to MarkZombieEdge

The `graph.Builder`'s `addZombieEdge` method is currently called during
funding transaction validation for the case where the funding tx is not
found. In preparation for moving this code to the gossiper, we export
the method and add it to the ChannelGraphSource interface so that the
gossiper will be able to call it later on.

graph/interfaces.go

@@ -85,6 +85,9 @@ type ChannelGraphSource interface {
 	// public key. channeldb.ErrGraphNodeNotFound is returned if the node
 	// doesn't exist within the graph.
 	FetchLightningNode(route.Vertex) (*models.LightningNode, error)
+
+	// MarkZombieEdge marks the channel with the given ID as a zombie edge.
+	MarkZombieEdge(chanID uint64) error
 }
 
 // DB is an interface describing a persisted Lightning Network graph.