Comments: Optimize the query for pending comments count in...

Comments: Optimize the query for pending comments count in `wp_notify_moderator()` for better performance on large tables.

Props sgoen.
Fixes #48514.
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git-svn-id: https://core.svn.wordpress.org/trunk@46678 1a063a9b-81f0-0310-95a4-ce76da25c4cd