Fix: not getting entire feed on url change

If the first entry of a feed changes its url feed.new_entries will
then contain all entries of the feed, even old, already seen ones.

find_new_entries_for only looked for the url of the first entry from the
last run to determine when the list of already seen entries starts. Not
finding the url, it did add all entries to new_entries.

Instead of using url, using id (which has a fallback to url) fixes this
bug.