Instance-Symbol child inheritance broken for WithConfigs and Socket nodes

Sub-nodes of an Instance node are supposed to be temporarily inherited by the right-most leaf node of the target Symbol's subtree - algorithm described here: http://scenejs.wikispaces.com/Instancing+Algorithm

This was broken for SceneJS.WithConfigs and SceneJS.Socket, which were using the old broken technique of passing the child array down through the scene traversal context stack. Result was that children were lost on descent into these nodes' children.

Fix them to use the new callback trick.