Send initial values of system variables in first OK packet

Currently, all drivers that need to know the values of some system variables must obtain this information by doing a separate SELECT. This adds latency to the connection creation.

The OK packet that's sent in response to successful authentication could contain initial values of the system variables that are tracked by session_track_system_variables. With this, drivers that support system variable tracking could read these values on startup and avoid the cost of the extra query.

An extension to this would be to provide some information about the status of the server to the drivers. One example of this would be the Threads_connected status variable that could help drivers that have connection pools to spread the load better if one of the servers in a cluster ends up with more connections than the others.