routing+channeldb: add AddProof method

Originally we adding the edge without proof in order to able to use it
for payments path constrcution. This method will allow us to populate
the announcement proof after the exchange of the half proofs and
constrcutrion of full proof finished.

channeldb/graph.go

@@ -507,6 +507,34 @@ func (c *ChannelGraph) HasChannelEdge(chanID uint64) (time.Time, time.Time, bool
 	return node1UpdateTime, node2UpdateTime, exists, nil
 }
 
+// UpdateChannelEdge retrieves and update edge of the graph database. Method
+// only reserved for updating an edge info after it's already been created.
+// In order to maintain this constraints, we return an error in the scenario
+// that an edge info hasn't yet been created yet, but someone attempts to update
+// it.
+func (c *ChannelGraph) UpdateChannelEdge(edge *ChannelEdgeInfo) error {
+	// Construct the channel's primary key which is the 8-byte channel ID.
+	var chanKey [8]byte
+	binary.BigEndian.PutUint64(chanKey[:], edge.ChannelID)
+
+	return c.db.Update(func(tx *bolt.Tx) error {
+		edges, err := tx.CreateBucketIfNotExists(edgeBucket)
+		if err != nil {
+			return err
+		}
+		edgeIndex, err := edges.CreateBucketIfNotExists(edgeIndexBucket)
+		if err != nil {
+			return err
+		}
+
+		if edgeInfo := edgeIndex.Get(chanKey[:]); edgeInfo == nil {
+			return ErrEdgeNotFound
+		}
+
+		return putChanEdgeInfo(edgeIndex, edge, chanKey)
+	})
+}
+
 const (
 	// pruneTipBytes is the total size of the value which stores the
 	// current prune tip of the graph. The prune tip indicates if the