build(deps): bump github.com/golang/protobuf from 1.4.2 to 1.4.3 (#4216)

Bumps [github.com/golang/protobuf](https://github.com/golang/protobuf) from 1.4.2 to 1.4.3.
- [Release notes](https://github.com/golang/protobuf/releases)
- [Commits](https://github.com/golang/protobuf/compare/v1.4.2...v1.4.3)
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