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{}

backend/chunker/chunker.go

@@ -262,6 +262,7 @@ func NewFs(name, rpath string, m configmap.Mapper) (fs.Fs, error) {
 		root: rpath,
 		opt:  *opt,
 	}
+	cache.PinUntilFinalized(f.base, f)
 	f.dirSort = true // processEntries requires that meta Objects prerun data chunks atm.
 
 	if err := f.configure(opt.NameFormat, opt.MetaFormat, opt.HashType); err != nil {

backend/crypt/crypt.go

@@ -186,6 +186,7 @@ func NewFs(name, rpath string, m configmap.Mapper) (fs.Fs, error) {
 		opt:    *opt,
 		cipher: cipher,
 	}
+	cache.PinUntilFinalized(f.Fs, f)
 	// the features here are ones we could support, and they are
 	// ANDed with the ones from wrappedFs
 	f.features = (&fs.Features{

backend/union/upstream/upstream.go

@@ -97,6 +97,7 @@ func New(remote, root string, cacheTime time.Duration) (*Fs, error) {
 		return nil, err
 	}
 	f.Fs = myFs
+	cache.PinUntilFinalized(f.Fs, f)
 	return f, err
 }