Maintain a list of blocked endpoints

[pcarrier]

Should it become necessary, we could block an endpoint abusing the service from registering again.

Ideally, ssh srv.us block $endpoint / ssh srv.us unblock $endpoint and ssh srv.us blocked to get the list. Though ssh srv.us -p 2222 would be fine and simpler.

Could be persisted in a flat text file reloaded on change, in badger, or in a central DB like cockroachdb.