Investigate LLMS_Processor_Course_Data improvements

**Is your feature request related to a problem? Please describe.**
For larger sites, the background processor (LLMS_Processor_Course_Data via the `llms_calculate_course_data` background process) can take too long and timeout, never "catching up" to finish processing.

**Describe the solution you'd like**
Investigate and improve the query or database indexes on tables like `wp_lifterlms_user_postmeta` to speed this up regardless of the site size.

**Describe alternatives you've considered**
Increasing the processing power of the site.

HS-259413