Should the 'counts' cache group be persistent?

[wjywbs]

I checked that the places storing data in the 'counts' cache group have proper way to delete the data in cache on updates. For example, wp_count_posts() stores the post count, and _transition_post_status() deletes the count. So could we change the 'counts' cache group to be persistent? This can reduce the repeated counting queries to the database.

Let's look

This needs a dev post when it goes in.

I would like to see some unit tests to support this change. Here is a draft for the make post for when that is ready.

## Persistent count caching in 4.8

The counts cache group has historically been excluded since [7986] fixed #6740. In the last 8 years, changes have made this no longer necessary. As such, counts is no longer a non-persistent group as of #XXXX. This should enable sites using a persistent cache to reduce the number of queries on certain page loads.

If you are are using the counts cache group in your code or are running a site with a persistant cache, you are highly encouraged to test this change.