dnsdist: Use LRU to clean the MaxQPSIPRule's store

dnsdist: Use LRU to clean the MaxQPSIPRule's store

This makes it possible to remove expired entries from the store
without having to scan more than a fraction of it. Entries are
ordered by their last usage, with least recently used ones at
the front, so we can stop scanning as soon as we find an entry
still valid. Even so, we will only consider a fraction of the
store during each pass to keep the cleaning fast, even with a
large store.