Update C++ module example for Oak API changes
Update C++ module example for Oak API changes

 - Encapsulate the response message in a GrpcResponse wrapper
   protobuf.
 - Shift to having the oak_main event loop in the module.

project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak