Only flag docs as loaded when they've actually changed

Only flag docs as loaded when they've actually changed

We already avoided the actual updates (by filtering the UPDATE
statement), but since we set the load date we'd trigger a change to
every page and kick it ouf of the caches even when not chagned, which is
wasteful. So instead only do that if something has changed. When it has,
we still reset that whole version of the docs since we want the load
date to be consistent across them.