Added columns for SSL in list sessions

To ensure, if a ssl is really ssl encrypted is sometime difficult to see,
only to rely on ssl is configured and get no error on connect,
is a bit few for a safetyness feeling.

Add two column where it can be visible, if a session is SSL encrypted.

Column names are

ssl_client -> show if connection is encrypted between client and maxscale
ssl_server -> show if connection is encrypted between maxscale and server

Values in the columns is empty or a concat of ssl-version and cipher.