Add a periodic check for "1 probe per IP" rule

[Martin Kolárik]

To do after #501.

Even though we already have a check for allowing just one probe per IP, it can fail in some special cases, such as network failures in retrieving the remote probes or a simple race condition when two clients connect each to a different server at once.

Given the distributed nature of the system, we can't really make the check 100% reliable. I suggest we simply add an extra periodic check and if we detect two probes with the same IP there, all except the first one get disconnected.