Werk #7326: "Update DNS cache" action is now really cleaning up the cache

As written in the documentation (https://mathias-kettner.com/cms_wato_hosts.html) Check_MK is keeping an internal DNS cache for the hosts that have configured no static IP address in Check_MK:

With the host name method Check_MK uses cached data in order to minimise repeated DNS requests during an Activate Changes – which is very important for accelerating the activation procedure. Furthermore, the cache ensures that a changed configuration can still be activated if the DNS stops working.

The catch is that Check_MK doesn't automatically notice the change to an address in DNS. For this reason, in the host details there is the button which deletes the entire DNS cache and forces a new resolution at the next Activate changes. This file is found under ~/var/check_mk/ipaddresses.cache in your instance, by the way. Deleting this file has the same effect as the button as described above.

The problem was that previous versions did not really delete the entire cache, but only updated it. We have now changed this to make the cache invalidation work as intended.

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