Make interim WAN cache key deactivation logic broader
* This is how it was before
d5aa846d8, which accidentally changed
the behavior. The WAN cache call in Setup.php was too coupled to
the ChronologyProtector cookies. It made the assumption that DB
positions and cookies are stored even when there are no replica
databases setup (which changed in that patch).
* Check the "UseDC" cookie instead, which is already set in exactly
the cases this logic should apply.
Bug: T185328
Change-Id: I4dd2a6ca6cdead0052d59be86e6030dbe01f141b