[feature request] support checkpoint/restore

[Ace-Tang]

Ⅰ. Issue Description

Implementing checkpoint/restore functionality, this functionality need a external software criu.

checkpoint

Checkpointing the address space and state of the entire process tree to a collection of “image” files.

restore

Restore a container from a checkpoint image.

pouch interface

1. pouch checkpoint create
   create a checkpoint from a running container using criu.

2. pouch checkpoint list
   list a container all checkpoints.

3. pouch checkpoint delete
   delete a container checkpoint.

4. pouch start --checkpoint
   restore a container from a checkpoint image.

Ⅱ. Describe what happened

Ⅲ. Describe what you expected to happen

Ⅳ. How to reproduce it (as minimally and precisely as possible)

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• pouch version (use pouch version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

[allencloud]

Could you add more details about what is checkpoint or the demand for checkpoint? @Ace-Tang