Add option for connection termination on `stop service`

Stopping the maxscale service (e.g., from maxctrl) does not terminate existing connections. This is rather counter-intuitive and confusing (stopping a module should prevent data flow through it completely). While it may be undesirable to change this default behaviour, it will be quite useful to get a config option that will achieve this - terminate existing connection on maxscale stop.

The same applies when a service is stopped - existing connections still work, which is confusing for a stopped service. Again, having an option to terminate connections when a service is stopped would be quite helpful.