[5.x] Fix static cache locking

[duncanmcclean]

This pull request attempts to fix a race condition with full-measure Static Caching, where if multiple requests come in for a URL at the same time, they might all try to write to the same file at the same time, causing mangled HTML, like seen in #10829.

I haven't been able to replicate the issue on my sandbox site, but I have been able to replicate it on a site @robdekort provided, using spatie/fork to perform concurrent requests.

To fix it, I've adjusted how we handle locking in the StaticCaching\Cache middleware, replicating how we do it elsewhere in the app. From my testing, it seems to fix the issue. 🤞

We made some changes to the static cache locking code in #10607. However, from memory, when I was last debugging this issue, I could reproduce it even after reverting that PR.

After some further testing, it seems like the original fix didn't actually fix the issue - I just got lucky. We've changed approach, see comment.

Fixes #10829.

[robdekort]

Oh this is really neat, thank you Duncan!

[jasonvarga]

We discovered the initial change in this PR didn't actually fix it.

After digging deeper we realized that the problem was the subsequent requests would wait for the first one to finish (the lock was working fine), but would then continue to write to cache the response. To fix it, after the lock was acquired we would try again to serve a cached version, since now there would be one.

[robdekort]

Thanks guys!