Improve multipart date check

Previously, if an SQL (or similar) date was in the DB (which may or may not still be
possible), only the time portion was returned. Now, test for the year
elsewhere in the multipart date string to make sure it's actually
multipart.
Fix for changed items with 'collections' appearing in 'unchanged'

Zotero_Item::setCollections() now just sets new collections in memory
instead of saving them itself, which was causing other changes in the
API import process to go unreported.
Fix sorting by date and item type in /top mode
Update cache on Atom XML cache mismatch