graph/db: let ForEachChannel take a context

graph/db/kv_store.go

@@ -404,8 +404,9 @@ func (c *KVStore) AddrsForNode(ctx context.Context,
 // 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.
-func (c *KVStore) ForEachChannel(cb func(*models.ChannelEdgeInfo,
-	*models.ChannelEdgePolicy, *models.ChannelEdgePolicy) error) error {
+func (c *KVStore) ForEachChannel(_ context.Context,
+	cb func(*models.ChannelEdgeInfo, *models.ChannelEdgePolicy,
+		*models.ChannelEdgePolicy) error) error {
 
 	return forEachChannel(c.db, cb)
 }