[34268] introduced cache priming for commentmeta, enabled by default. To
ensure performance on single post pages - where commentmeta is most likely
to cause performance issues - we disable up-front cache-priming. Instead, we
prime commentmeta caches for all comments in the loop the first time
`get_comment_meta()` is called on the page.
Props bradt, dd32, wonderboymusic, boonebgorges.
Fixes#16894.
git-svn-id: https://develop.svn.wordpress.org/trunk@34270 602fd350-edb4-49c9-b593-d223f7449a82
504deb52fc