combine: fix errors with backends shutting down while in use

Before this patch backends could be shutdown when they fell out of the
cache when they were in use with combine. This was particularly
noticeable with the dropbox backend which gave this error when
uploading files after the backend was Shutdown.

    Failed to copy: upload failed: batcher is shutting down

This patch gets the combine remote to pin them until it is finished.

See: https://forum.rclone.org/t/rclone-combine-upload-failed-batcher-is-shutting-down/32168

backend/combine/combine.go

@@ -145,6 +145,7 @@ func (f *Fs) newUpstream(ctx context.Context, dir, remote string) (*upstream, er
 		dir:            dir,
 		pathAdjustment: newAdjustment(f.root, dir),
 	}
+	cache.PinUntilFinalized(u.f, u)
 	return u, nil
 }