Fix border wrapping on folia

[Warriorrrr]

This PR fixes wrapping on folia by using getElevation non-blockingly, and as a free bonus, prevents doing a sync chunk load on paper servers.

[pop4959]

Some initial comments & requests:

• Can you break this up into separate PRs for fixing wrap, and breaking this up into per-player tasks (logically, they're separate changes, and it'll make things slightly easier to review). I think both are reasonable changes to make.
• I haven't checked again in detail, but I do not believe there is any advantage to schedule the tasks differently between Paper and Folia. If going task per player, it's probably fine to just do that on both platforms. I think running a single global task was more of a micro-optimization on my part originally. The task is making a loop over all players anyway.
• I know there was some back and forth about getElevation in chunky, and whether that should return CompletableFuture. Originally the idea was to squeeze in this Folia-specific managed block code to avoid a whole refactor, but seeing as we're doing it anyway here, I'd rather not have ChunkyBorder wrap around all of this just to create a CompletableFuture out of it when that's the only thing getElevation is being used for in the first place. So we might as well just make that change in Chunky at this point to avoid all of this extra wrapper code (including that in BukkitBorderCheckTask which should not be necessary then).

Aside from that, the changes here look generally good to me at a glance. Thanks for cooking this draft up.

[pop4959]

Good catch here

[pop4959]

Thanks for addressing my comments, this is more in-line with what I was expecting for a refactor in this part of the code. Pending the merging of pop4959/Chunky#424 this looks good.