dnsproxy: Delete expired cache entries when they expire

dnsproxy: Delete expired cache entries when they expire

The DNS cache has logic to never replace cached data if it already exists.
However, this only works well if we actually delete the data parts
of the cache entries as they expire.

This patch creates a cache_enforce_validity() function, which will
delete the data portion of cache entries if they are expired.
With this in place, consecutive lookups of the same name can get
cached again.