refactor: move graph related DB code to graph/db from channeldb

This is a pure refactor commit. It moves over all the graph related CRUD
code from `channeldb` to `graph/db`.

graph/notifications.go

@@ -10,7 +10,7 @@ import (
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/go-errors/errors"
-	"github.com/lightningnetwork/lnd/channeldb"
+	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
@@ -318,7 +318,7 @@ func addToTopologyChange(graph DB, update *TopologyChange,
 
 	// Any node announcement maps directly to a NetworkNodeUpdate struct.
 	// No further data munging or db queries are required.
-	case *channeldb.LightningNode:
+	case *graphdb.LightningNode:
 		pubKey, err := m.PubKey()
 		if err != nil {
 			return err