dnsproxy: Add a "invalidate cache" function

dnsproxy: Add a "invalidate cache" function

When something changes in the network topology, we need to invalidate the cache.
This first implementation will delete all entries (which could have been done
more efficiently), but in later patches in this series we'll enhance this logic
to only delete the cached data, not the names. With keeping the names,
we can then re-lookup popular data in the new topology.