Update node via set node

[nyanpassu]

This PR introduce a feature to enable modify a node endpoint via SetNode rpc method.

What I do and how I archived the goal:

1. the grpc protocol is updated, a string field of endpoint is added to the message SetNodeOptions, so as to carry value of new endpoint
2. the transform from pb.SetNodeOptions to types.SetNodeOptions is modified to enable passing value of new endpoint
3. the method of calcium is modified to distinguish whether or not to update endpoint, if the value of SetNodeOptions.Endpoint is "" the current endpoint value of node is kept, or else the value of node endpoint is set to the new value pass in by SetNodeOptions.Endpoint
4. unit test is updated to ensure our goal