Refactors locks in AdsClient to avoid requirements of universal locks

[thesuzerain]

This restructure allows us to avoid having to lock the entire AdsClient, but only restricts locks to what we specifically need- in this case, HttpCache.

This allows:

• stopping an HTTP query from locking the entire ads client.
• In turn, this lets us move logic out of the FFI layer that we need to be able to resolve 'instantly' (the shutdown logic, or any future fire-and-forget requests to the background worker)

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[thesuzerain]

This impl is a bit over the top and something I may not keep, I just didn't like the Option<...<Option<...>> and the nested structures it required to get locks. This lets us treat the double option as a single option. I think this is uglier actually. I'm considering a couple options before I PR this:

• Arc<Mutex<Option<HttpCache>>> and get a lock every time even if HttpCache is unset.
• Option<Arc<Mutex<Option<HttpCache>>>> and keep the nested Option handling.

Either way this code snippet is not the critical part of this refactor. However, we do need to pass this mutex 'up and out' to the AdsClient test shutdown_does_not_require_http_client_lock