_get_non_cached_ids() should use wp_cache_get_multi(), if available.

[ryan]

Add wp_cache_get_multi() to all cache backends and conditionally use it in _get_non_cached_ids() so that post objects can be fetched from cache in parallel.

[nacin]

Original ticket for wp_cache_get_multi(), #20875.

[wonderboymusic]

Patch gets the ball rolling - wp_cache_get_multi() is in #20875

[chriscct7]

#20875 (a dependency for this ticket) is closed, thus closing this as maybelater as well

[jeremyfelt]

Reopening along with #20875 for a hopeful future.