wp_start_object_cache() should be called within blog switching methods instead of wp_cache_init()

[prettyboymp]

The switch_to_blog() and restore_current_blog() functions call wp_cache_init() directly, which causes the object cache instance to be recreated. These methods should instead be calling the wp_start_object_cache() method which correctly checks whether wp_cache_init() or wp_cache_reset() should be used.

[scribu]

Related: #14992

[scribu]

(In [16359]) Use wp_start_object_cache() in switch_to_blog(). Props prettyboymp. Fixes #15361

[ryan]

We don't really need to re-add global and non-persistent groups or do those file and function exists checks. Maybe switch_to_blog() should directly call wp_cache_reset() if it exists otherwise do wp_cache_init().

[Denis-de-Bernardy]

Suggesting we revert the changeset, for this reason:

​http://core.trac.wordpress.org/ticket/14992#comment:7

[ryan]

(In [16597]) Revert [16359]. Deal with it in 3.2. see #15361

[nacin]

I think this is mostly handled now by #21434.