Try caching get_remaining_tickets() in APCu to allow tickets to be 'reserved' during checkout, avoiding race condition

[dd32]

Draft PR due to inability to properly test this right now, and unsure if it's the proper direction forward or worth it.

Ref: https://wordpress.slack.com/archives/C08M59V3P/p1712066063709249

WordCamp Sylhet ran into an issue where WordCamp tickets were oversold, due to a race condition during the checkout process.

The ticket are counted on every pageload, and in the event of a few hundred people attempting to checkout at the same instant, resulted in multiple tickets inserting tickets after the initial do we have available tickets check passed.

This PR attempts to add some caching/locking using APCu to the process.

This appears to work at first glance, but is not entirely complete.

• failed payments locks a ticket for up-to 10 minutes (possibly a good thing?) (fixable)
• changing the available tickets number doesn't reflect for the up-to 10 minutes (fixable)
• Hard to test due to low requests/second sandbox throughput

While writing this PR, I've noticed something that was rather odd.
It appears that some of the order verification is checked after the check for errors, resulting in some of the validation being skipped during checkout, although it seems duplicative.. ba27a3a moved that and I've split it into #1295

[dd32]

I'm closing this, as I don't think this actually resolved anything. I suspect something like #1297 would be better suited.