[Serve] Feature request: support num_nodes for the Controller

[HysunHe]

Currently, the controller node is a sington, which may cause single point of failure.

Because the controller node has the function of proxying user requests to backend serving nodes besides its management role, the failure of it may cause the caller applications fail on calling the backend services.

Multiple controller nodes are useful.

If there are multiple controller nodes, even they are independent from each other, user can take the advantage of the high availability from them by, for example, employing load balancer or leveraging DNS round-robin functions.