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

cmd/cmd.go

@@ -321,6 +321,12 @@ func Run(Retry bool, showStats bool, cmd *cobra.Command, f func() error) {
 		}
 	}
 
+	// clear cache and shutdown backends
+	cache.Clear()
+	if lastErr := accounting.GlobalStats().GetLastError(); cmdErr == nil {
+		cmdErr = lastErr
+	}
+
 	// Log the final error message and exit
 	if cmdErr != nil {
 		nerrs := accounting.GlobalStats().GetErrors()