resultset => something that describes the results of a query/execution. This may contain more than 1 resultset
server_diagnostics => something that is not server-specific. When we develop #60, the client will likely have to generate diagnostics that should go in this structure
server_error => rename with the logic above
server_category => something that allows introducing MySQL and MariaDB specific categories. Error codes specific to MySQL should be removed from here. See #108.
Consider dropping field::emplace_xxxx (I don't think they add any value).