DNS resolution for validator lists

[carlhua]

Validator list site are configured by specifying a URL from which the validator list can be retrieved. The URL can specify either a hostname or an IP address.

If a hostname is specified then we should resolve it every time we need to perform a check against that site; if the resolver response includes a single IP, then we can just use it normally: attempt to connect to that IP and retrieve the list. Store the IP address and the success/failure status in the object and produce a log entry.

If the resolver response includes multiple IPs we need to use more advanced logic. Resolve the hostname again and:

If the previous attempt was successful and if the previously used IP is still valid use that, following the "single IP" logic above.
If this isn't the first attempt and the previous attempt failed, remove the failed IP from the list of resolved addresses if it still exists. Select an IP at random from the remaining choices and follow the "single IP" logic above.

Exported from RIPD-1671