for performance: change JSONBCodec format to BinaryFormatCode.

[piyongcai]

just for performance. because json.Unmarshal need []byte parameter

[jackc]

In general, I don't want to merge any performance optimizations without a benchmark that shows the improvement.

That's especially true in this case because the text format was chosen on purpose. PostgreSQL (at least historically) has a significant performance penalty when returning text-like types as binary. See https://github.com/jackc/pg_text_binary_bench.