Prevent IP / alt IP conflicts

[MartinKolarik]

Depending on timing, it is currently possible we'd register a single alt IP for multiple probes, or we'd register the same IP once as primary and once as alt. It seems like a good idea to add trigger checks for these:

• changing primary IP - it doesn't exist in altIps yet
• changing altIps - it doesn't exist in either primary IPs or altIps

Additionally, we should add a check here that if it fails to update, it's as if the probe wasn't found at all.