Add support for re-authentication

Currently it is possible to authenticate, but not to re-authenticate, that is, to change used credentials without reconnecting. It is trivial to support that in the connection between the client and MaxScale, but any changes there have to be reflected in the connections between MaxScale and the backend servers. That can either be supported by generating a COM_CHANGE_USER packet or simpler by just closing the backend connections and reconnecting with new MariaDB credentials.