Add the blog id to the cache keys for multisite installs.
Use wp_cache_switch_to_blog() instead of wp_cache_init() in switch_to_blog().
Use wp_cache_switch_to_blog() instead of wp_cache_reset() in wp_start_object_cache().
Deprecate wp_cache_reset().
This avoids the many queries needed to re-prime the cache after switch_to_blog() on multisite installs using the default cache backend.
fixes#21434
git-svn-id: http://core.svn.wordpress.org/trunk@21403 1a063a9b-81f0-0310-95a4-ce76da25c4cd