Update protobuf

[ttreptow]

Hi, I'm evaluating/prototyping this project for internal use at the company I work for and I have made a couple of updates that might be of interest:

First off, switching fully to google.golang.org/protobuf and updating the proto generation accordingly.

Secondly (and this one I'm unsure about), removing all the vendored packages. I haven't worked with vendoring much so I don't know what the use case is, but the application builds fine without them with a modern version of Go so they seem kind of redundant. However there could be some good reason for having them there that I'm unaware of.

I have both of these changes in this branch but I could split them if needed.