Draft: Callback Scopes and Method Reply Dispatch

There are still a few races with caching that can happen because replies to initially populate a cache and signals to update it aren't ordered with the usual async API. This branch allows callbacks for method returns and uses them to fix this race.

It also adds support for making dbus calls or doing other slow tasks from within callbacks, which was at a risk for deadlock previously, and allows restoring the previous callback parallelism if sequential processing is not needed.

This branch also adds knobs to dbus_proxy to remove many of the extra methods that it and async have added (or to split them off to an "advanced proxy" similar to how zbus splits off its blocking proxies).

Fixes #180 (closed).