Properly invalidate the cache for `wp_count_posts()` on insert, trash, or when...

Properly invalidate the cache for `wp_count_posts()` on insert, trash, or when transitioning `post_status` inside of `_transition_post_status()`. Introduces `_count_posts_cache_key()`. Adds unit tests.

Props mark8barnes, for bringing this to our attention in an initial patch.
Fixes #21879.


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git-svn-id: https://core.svn.wordpress.org/trunk@26954 1a063a9b-81f0-0310-95a4-ce76da25c4cd