load-test-framework: go generated protobuf code broken

In #304, the [loadtest.pb.go file](https://github.com/GoogleCloudPlatform/pubsub/commits/master/load-test-framework/go_src/internal/genproto/loadtest.pb.go) was overwritten with a new format. This changed the package name and also many methods, which subsequently broke the loadtesting framework.

Some solutions are to rewrite the existing go loadtesting code to use the new generated proto format, or revert back to the old one.