refactor(protobuf): [Breaking] Use google.golang.org/protobuf

[lrh3321]

Problem

1. github.com/golang/protobuf and https://github.com/gogo/protobuf were deprecated.
2. XXX_DiscardUnknown, XXX_Marshal, XXX_Merge, XXX_Size, XXX_Unmarshal methods for structs in badgerpb4.pb.go are removed.

Solution

1. Use google.golang.org/protobuf/cmd/protoc-gen-go to generate badgerpb4.pb.go
2. Add Size, Marshal, MarshalTo, MarshalToSizedBuffer, Unmarshal methods to pb.KV, pb.DataKey, like protoc-gen-gogofaster does.
3. Replace the imports

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