HostDB caching of SOA responses doesn't honor TTL

HostDB categorizes all DNS responses as "failed" and "not failed", "failed" being any response that doesn't have an answer to the question sent. This means that SOA responses are considered "failed" and therefore the response isn't cached for the TTL defined in the SOA record. "failed" responses TTL are configured by "proxy.config.hostdb.fail.timeout" (default to 0). With the value set to default the record will be immediately considered stale and require a lookup. To fix this we should have the "failed" response honor the TTL if the response had a TTL defined.