Use wp_cache_add() instead of wp_cache_set() in update_meta_cache()

[ryan]

update_meta_cache() already does a wp_cache_get() on each object_id and excludes those that are already in the cache from further processing. Later on it does a wp_cache_set(). This can be a wp_cache_add() instead. Using adds is preferred policy when priming the cache (as opposed to updating the cache when something is changed, which should use set) and can improve cache performance in certain scenarios since an add does not force invalidation.

[scribu]

Related: #14817

[ryan]

(In [16437]) Use wp_cache_add() instead of wp_cache_set() in update_meta_cache(). fixes #15452