Add default database to session status output

Original title: Can the list sessions include "database" and "protocol" columns for the sake of Persistent Connections?

Folks,

Working with the MaxScale Persistent Connections, I'm missing a way to make sure all the connections the MaxScale is receiving can be used. Maybe there is another way to look at that piece of information I'm looking for, but, using the `list sessions` command with the MaxCtrl is a pretty convenient way of retrieving that.

Researching a little bit, I found on the public documentation that:

Please note that because persistent connections have previously been in use, they may give a different environment from a fresh connection. For example, if the previous use of the connection issued "use mydatabase" then this setting will be carried over into the reuse of the same connection. For many applications, this will not be noticeable since each request will assume that nothing has been set and will issue fresh requests such as "use" to establish the desired configuration. In exceptional cases, this feature could be a problem.

I'm not saying that I have that issue, but, if the `maxctrl list sessions` could include the database (if the connection does a <use db>) and the protocol (which by now will always by the mariadbbackend). This way we can look at that and tell, connections are being reused.

What do you think? Thanks!