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`.

routing/graph.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 
 	"github.com/btcsuite/btcd/btcutil"
-	"github.com/lightningnetwork/lnd/channeldb"
+	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/routing/route"
 )
@@ -15,7 +15,7 @@ type Graph interface {
 	// ForEachNodeChannel calls the callback for every channel of the given
 	// node.
 	ForEachNodeChannel(nodePub route.Vertex,
-		cb func(channel *channeldb.DirectedChannel) error) error
+		cb func(channel *graphdb.DirectedChannel) error) error
 
 	// FetchNodeFeatures returns the features of the given node.
 	FetchNodeFeatures(nodePub route.Vertex) (*lnwire.FeatureVector, error)