Caching of unresolved domains

[ppcad]

The domain resolver caches unresolved domains for increased performance.
However, a domain can be unresolved due to a timeout, the domain being unknown or the domain not having a valid syntax.

This can lead to domains not being resolved for the configured max caching days if the resolver happens to timeout when it's cached.

Those cases could be handled differently:

• Invalid domains don't need any caching, since there is never a request
• Domains with timeouts could be cached for a shorter period of time or not at all
• Unknown domains can be cached as usual, since they are unlikely to change frequently and they will leave the cache eventually anyways