Add support for MariaDB connection redirection

[mirromutth]

In the Ok packet of MariaDB, the session state information may contain redirect_url and the driver can keep the old connection, or redirect to the new URL.

Supporting redirection mechanism is helpful for proxy-based scenario. It is more like HTTP redirects or Oracle redirected connections. Clients get the servers’ address from proxies and connect to servers transparently without latency and computing resource wasted.

It is based on a common feature of MySQL and MariaDB, #227

See also https://jira.mariadb.org/browse/MDEV-15935 and https://mariadb.com/kb/en/ok_packet/#session_track_system_variables .