SALAD: add support for read-only consoles

It would be great if multiple clients could connect to SALAD and get the logs from the same machine in a read-only way. I can see it being useful in these scenarios:

• Dashboard: allow viewing the logs coming from any machine
• Reliability: Gitlab jobs do not need write access, and we keep hitting issues where salad complains that there is already a client connected

This could be achieved by opening a second tcp port for read-only operations.