This change ensures we call the Shutdown method on backends when
they drop out of the fs/cache and at program exit.
Some backends implement the optional fs.Shutdowner interface. Until now,
Shutdown is only checked and called, when a backend is wrapped (e.g.
crypt, compress, ...).
To have a general way to perform operations at the end of the backend
lifecycle with proper error handling, we can call Shutdown at cache
clear time.
We add a finalize hook to the cache which will be called when values
drop out of the cache.
Previous discussion: https://forum.rclone.org/t/31336
Martin Czygan