fs: Pin created backends until parents are finalized

This attempts to solve the backend lifecycle problem by

- Pinning backends mentioned on the command line into the cache
  indefinitely

- Unpinning backends when the containing structure (VFS, wrapping
  backend) is destroyed

See: https://forum.rclone.org/t/rclone-rc-backend-command-not-working-as-expected/18834

backend/cache/cache.go

@@ -385,6 +385,7 @@ func NewFs(name, rootPath string, m configmap.Mapper) (fs.Fs, error) {
 		cleanupChan:      make(chan bool, 1),
 		notifiedRemotes:  make(map[string]bool),
 	}
+	cache.PinUntilFinalized(f.Fs, f)
 	f.rateLimiter = rate.NewLimiter(rate.Limit(float64(opt.Rps)), opt.TotalWorkers)
 
 	f.plexConnector = &plexConnector{}