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

vfs/vfs.go

@@ -234,8 +234,8 @@ func New(f fs.Fs, opt *vfscommon.Options) *VFS {
 
 	// Pin the Fs into the cache so that when we use cache.NewFs
 	// with the same remote string we get this one. The Pin is
-	// removed by Shutdown
-	cache.Pin(f)
+	// removed when the vfs is finalized
+	cache.PinUntilFinalized(f, vfs)
 
 	return vfs
 }
@@ -293,9 +293,6 @@ func (vfs *VFS) Shutdown() {
 		return
 	}
 
-	// Unpin the Fs from the cache
-	cache.Unpin(vfs.f)
-
 	// Remove from active cache
 	activeMu.Lock()
 	configName := fs.ConfigString(vfs.f)