dnsdist: Add a new chain of rules triggered after cache insertion

dnsdist: Add a new chain of rules triggered after cache insertion

The general idea is to be able to store the unedited version into
the cache while delivering a different version to the actual client.
This is useful when one is sending different answers to different
clients, like when dealing with abuse traffic, but still want to be
able to cache the initial response from the backend.
We already have a chain of rules that are triggered after a cache-hit,
but until now we lacked the ability to trigger after getting the
response corresponding to a cache-miss.