lib/setup: restore sanity to rcache handling

lib/setup: restore sanity to rcache handling

$CFG->rcache is safe to keep, even as we are reading the config table.

This is because whenever we update the config table, all the rcache
plugins either mark the cache as dirty or they update the cache.

The reason we want to be able to preserve this is that rcache settings
in CFG should usually be set in config.php rather than in the config
table.

With this fix, we support both control points.