Rewrite context managers (i.e. in caches) with AsyncExitStack

[dolamroth]

Use the following logic: https://github.com/agronholm/apscheduler/blob/master/src/apscheduler/_schedulers/async_.py#L159-L166

basically, enter the exit stack already in __aenter__(), and if all goes well, use the .pop_all() method to extract a new exit stack with all the callbacks for __aexit__() to unwind
this makes sure that the exit stack still unwinds even if there's an exception in __aenter__()
I use this technique everywhere when the logic of __aenter__() is nontrivial
if you don't do this, and __aenter__() raises an exception after you've entered a cancel scope, the entire task is broken beyond repair
``` (c) agronholm