Update protoc to v3.9.1, protoc-gen-go to v1.3.1

[tdmanv]

Dependency changes

• https://github.com/protocolbuffers/protobuf/compare/v3.5.1...v3.9.1
• https://github.com/golang/protobuf/compare/v1.2.0...v1.3.2

Generation

Generated in golang:1.12.7 using:

docker run -it --volume $(pwd):/go/src/github.com/container-storage-interface/spec --workdir /go/src/github.com/container-storage-interface/spec golang:1.12.7 bash

logs: https://gist.github.com/tdmanv/7244b55e0238a38f35be1b1dfd667358

References

Fixes https://github.com/container-storage-interface/spec/issues/377

[jieyu]

This LGTM. @jdef @saad-ali can you take a look?

[saad-ali]

/lgtm /approve