NewServer returns a new instance of Server, set to accept requests on addr.

StartServer makes a new Server and gets it to start listening on the given net.Listener.

Request wraps the request protobuf (xds/LRS) and error received by the Server in a call to stream.Recv().

Response wraps the response protobuf (xds/LRS) and error that the Server should send out to the client through a call to stream.Send().

Server is a fake implementation of xDS and LRS protocols.