Variables for cache backend classes were consolidated

Previously, to change the cache backend for a specific cache bin, you would use a variable like cache_class_*, e.g.

$conf['cache_class_cache_page'] = 'MyCustomCache';

Now that would read

$conf['cache_classes']['cache_page'] = 'MyCustomCache';

The default cache bin was previously defined as a separate variable like this

$conf['cache_default_class'] = 'MyCustomCache';

Instead of a separate variable, the cache backend of the default bin "cache" is now used as the fallback, which can be set like this

$conf['cache_classes']['cache'] = 'MyCustomCache';