Refactor `protobuf` with other header-only serialization library like `flatbuffers`

[RobinQu]

Currently libproto.a is always needed as only library users have to link with.

Possible alternatives: https://github.com/google/flatbuffers

[RobinQu]

It's in low priority. But as so many things are concerned, it should be noted.