Add helper get_total_objects_for_query_from_db() which counts directly from the DB

[rebeccahum]

Description of the Change

We're noticing that on very large sites, the query to count the total objects in get_total_objects_for_query() dies. This PR adds an abstracted helper method get_total_objects_for_query_from_db() that breaks it down using wp_count_posts to count the posts directly from the DB.

Alternate Designs

N/A.

Possible Drawbacks

A bit more processing during indexing if the count query has died, but at least the user will have an idea of the number of posts, rather than "0".

Verification Process

On a verrrry large site where it says 0 as the total objects, i.e. Processed x/0, apply the PR and expect it to return a number.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed: Add an alternative way to count total posts on larger DBs during indexing

Credits

Props @rebeccahum

[rebeccahum]

@felipeelia Thanks for your feedback! All have been addressed now.