Add ability to stop/start Listener

In MaxScale we can (remotely) start/stop certain components like services; however, MaxScale does lacks the ability to start/stop a listener.

The main expected feature here is that all existing client connections (and related server connections) should be terminated - either immediately or after completing the current query; it would be nice to have a config option to chose termination mode.

This feature is useful in all cases when we want to ensure clients connect to the proper servers; a typical one is a service reconfiguration when all existing connections will still point to the old servers, which we want to avoid; currently the only way of doing this is a complete restart of the daemon, which may be undesired when done frequently or when MaxScale is engaged with multiple separate clusters.

It is understood that if the admin interface or REST API listener gets stopped, the corresponding management channel becomes unavailable.