dnsdist: Clean up expired entries from all the packet cache's shards

dnsdist: Clean up expired entries from all the packet cache's shards

Otherwise we might remove enough entries from the first shards only
and stop there, which means that the other shards might remain full.
This might be fine if we clean up often enough since the next cleaning
run will start with the remaining shards, but that's sub-optimal when
we are often nearly full because it will prevent new entries from being
inserted in the shards that are full.