Cache API: Introduce `wp_cache_get_multi()`.

Many caching backend have support for multiple gets in a single request. This brings that support to core, with a compatability fallback that will loop over requests if needed.

Fixes: #20875.
Props: nacin, tollmanz, wonderboymusic, ryan, jeremyfelt, spacedmonkey, boonebgorges, dd32, rmccue, ocean90, jipmoors, johnjamesjacoby, tillkruess, donmhico, davidbaumwald, SergeyBiryukov, whyisjake.


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git-svn-id: https://core.svn.wordpress.org/trunk@47711 1a063a9b-81f0-0310-95a4-ce76da25c4cd