Use member NodeHandle in action clients.

Currently services and topics are already using the member NodeHandle instance,
but not the action clients.

This is relevant for two reasons:
- Consistency in the resulting ROS API namespace (everything in the same namespace).
- Consistency in the spinning policy. All services, topics and actions will be spinned
  by the same NodeHandle, and whatever custom (or not) spinners and callback queues it
  has associated.