Commit the filesystem layer during checkpoint

[huikang]

• The aufs layer is commited during checkpoint
• criu image path and image ID is persisted to the container config file

Signed-off-by: Hui Kang hkang.sunysb@gmail.com

[huikang]

To test this PR

1. Create a container

./docker-1.10.0-dev run -d busybox:latest /bin/sh -c 'i=0; while true; do echo $i >> /foo; i=$(expr $i + 1); sleep 1; done'

1. Create a directory or file for the container, e.g., /root/test
2. Checkpoint the container with --leave-running=true

./docker-1.10.0-dev checkpoint --leave-running=true <container ID>

1. Delete the directory or the file, /root/test
2. Stop the container
3. Restore the container with --force=true and you will see the file /root/test because the restored container uses the image layer during checkpoint
   (run this comment twice to restore the container due to some network bridge bug in checkpoint)

./docker-1.10.0-dev restore --force=true

[huikang]

@boucher @avagin, could you take a minute to review this PR? Thanks.