Imporve the performance of writing chunks to connection.

Feature Request

Is your feature request related to a problem? Please describe:

In https://github.com/pingcap/tidb/blob/7bfa896/server/conn.go#L1269, we have a chunk to write to connection, but we dump a chunk to mysql protocol row by row.

Describe the feature you'd like:

Now we can vectorize dumpBinaryRow and dumpTextRow by converting a chunk to network buffer column by column.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: