Optimize stat logging

[yscik]

A few follow-up optimizations and improvements to look at

All stats:

• Maybe make the stat names shorter?

Search impressions stats (#2771):

• Batch writing the events to DB
• Maybe reduce request payload size? Could compress into one event with multiple post IDs? Eg { job_listing_impression: [ 11, 12, 13, 14 ] }
• Debounce: send if 10 or so events are in the queue, don't wait for scrolling to stop
• Check if the JS is blocking rendering and if anything could be moved off the main thread
• Fix: investigate why sometimes a few listings are missed — registered as viewed but not as sent
• Maybe move CSS selectors in the observers to PHP so it's filterable if a theme is using different classes?
• Optimize page_has_jobs_shortcode check

[github-actions]

Closed by #2785.