MEDIUM: cache: Avoid going over duplicates lists too often

MEDIUM: cache: Avoid going over duplicates lists too often

The secondary entry counter cannot be updated without going over all the
items of a duplicates list periodically. In order to avoid doing it too
often and to impact the cache's performances, a timestamp is added to
the cache_entry. It will store the timestamp (with second precision) of
the last iteration over the list (actually the last call of the
clear_expired_duplicates function). This way, this function will not be
called more than once per second for a given duplicates list.