Provide more flexibility to the `get_{$adjacent}_post_where` hook

[danburzo]

I'm working on getting the previous/next post for a Custom Post Type based on a certain postmeta value, and the get_{$adjacent}_post_where seems the appropriate hook for this. However, it currently receives the whole WHERE clause as a parameter. IMO it would be ideal to be able to override just the WHERE p.post_date $op %s AND p.post_type = %s part and keep the other (useful ones) intact.

This could potentially be solved in a non-breaking way by sending $where as it stands before concatenation with WHERE p.post_date $op %s AND p.post_type = %s, so that authors can neatly plugin a postmeta condition.

[danburzo]

The hook is defined here: tags/4.8.1/src/wp-includes/link-template.php#L1716​
(I do realize I need to tap into get_{$adjacent}_post_join as well)