offline keys in service discovery

[0pcom]

https://sd.skycoin.com/api/services?type=visor

[2025-06-22T10:31:06.72557439-05:00] INFO [public_autoconnect]: Fetching public visors
[2025-06-22T10:31:07.100674812-05:00] DEBUG [public_autoconnect]: Found public visors public visors=1
[2025-06-22T10:31:08.948642814-05:00] DEBUG [public_autoconnect]: Found visors to connect to total=1
[2025-06-22T10:31:09.224853195-05:00] DEBUG [public_autoconnect]: No uptime tracker data for remote visor error="cannot iterate over: null" pk=03aac6564f5a0c5f4c7c947b9421c20a108b43b24112648dd5500126bd2157d08b

This should not be dealt with using a monitor implementation. We just need re-registration logic and de-registration logic to be implemented correctly. The visor should re-register itself in the service discovery every 5 minutes with the services it's running, and those entries should be expired if re-registration window is exceeded without re-registration.