invoicesrpc: remove invoicerpc server's access to ChannelGraph pointer

Define a new GraphSource interface required by the invoicerpc server and
remove its access to the graphdb.ChannelGraph pointer. Add the new
invoicesrpc.GraphSource interface to the GraphSource interface
and let DBSource implement it.

routing/blindedpath/blinded_path.go

@@ -2,6 +2,7 @@ package blindedpath
 
 import (
 	"bytes"
+	"context"
 	"errors"
 	"fmt"
 	"math"
@@ -42,7 +43,8 @@ type BuildBlindedPathCfg struct {
 
 	// FetchChannelEdgesByID attempts to look up the two directed edges for
 	// the channel identified by the channel ID.
-	FetchChannelEdgesByID func(chanID uint64) (*models.ChannelEdgeInfo,
+	FetchChannelEdgesByID func(ctx context.Context,
+		chanID uint64) (*models.ChannelEdgeInfo,
 		*models.ChannelEdgePolicy, *models.ChannelEdgePolicy, error)
 
 	// FetchOurOpenChannels fetches this node's set of open channels.
@@ -643,7 +645,9 @@ func getNodeChannelPolicy(cfg *BuildBlindedPathCfg, chanID uint64,
 
 	// Attempt to fetch channel updates for the given channel. We will have
 	// at most two updates for a given channel.
-	_, update1, update2, err := cfg.FetchChannelEdgesByID(chanID)
+	_, update1, update2, err := cfg.FetchChannelEdgesByID(
+		context.TODO(), chanID,
+	)
 	if err != nil {
 		return nil, err
 	}