Make clients caches thread safe.

[CharString]

@SilviaAmAm Another thing that just sprang to mind: functools.cache and functools.lru_cache are thread-safe. But I'm unsure if APIClient is; I doubt it's safe to have separate threads do a with client(): ... when they're the same APIClient instance... Now that I've written it down, I even highly doubt it. 😅.

It it isn't a fix is fairly simple. Have client() always return a freshly built instance, "move up" the cache a step.

Originally posted by @CharString in maykinmedia/open-archiefbeheer#881 (comment)

See implementation in maykinmedia/open-archiefbeheer@d99e033