fs/cache: make sure we call the Shutdown method on backends

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

fs/cache/cache.go

@@ -25,6 +25,11 @@ func createOnFirstUse() {
 		c = cache.New()
 		c.SetExpireDuration(ci.FsCacheExpireDuration)
 		c.SetExpireInterval(ci.FsCacheExpireInterval)
+		c.SetFinalizer(func(value interface{}) {
+			if s, ok := value.(fs.Shutdowner); ok {
+				_ = fs.CountError(s.Shutdown(context.Background()))
+			}
+		})
 	})
 }