Reduce queries by wp_count_posts()

[josephscott]

The new function wp_count_posts() needs to do a query for each combination of post_type and post_status. By doing a 'GROUP BY post_status' for each post_type request we can provide more data with one query.

Here are patches that make this change and update the dashboard to make use of this new feature.

One other thing I came across in the dashboard is a query for the number of approved comments. The dashboard patch removes this query because that data isn't used anymore. If it does need to come back it should use the get_comment_count() function instead.

(In [6808]) Reduce queries by wp_count_posts(). Props josephscott. fixes #5820