graph/db: let ForEachChannel take a context

graph/db/interfaces.go

@@ -158,7 +158,7 @@ type V1Store interface { //nolint:interfacebloat
 	// NOTE: If an edge can't be found, or wasn't advertised, then a nil
 	// pointer for that particular channel edge routing policy will be
 	// passed into the callback.
-	ForEachChannel(cb func(*models.ChannelEdgeInfo,
+	ForEachChannel(ctx context.Context, cb func(*models.ChannelEdgeInfo,
 		*models.ChannelEdgePolicy,
 		*models.ChannelEdgePolicy) error) error