Bump github.com/golang/protobuf from 1.4.0 to 1.4.3

Bumps [github.com/golang/protobuf](https://github.com/golang/protobuf) from 1.4.0 to 1.4.3.
- [Release notes](https://github.com/golang/protobuf/releases)
- [Commits](https://github.com/golang/protobuf/compare/v1.4.0...v1.4.3

)

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