Don't erase the last DNS name we have for an IP address

This is in a sense wrong, but having a stale data is likely better than
having none. In the future, when we have a trustworthiness flagging for
the names, we might flag it as stale.
1 job for dns-cache in 2 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #6757