Commit running session and push image to dedicated registry

[kyujin-cho]

Let's create a new API to automatically commit running session and publish built artifact to designated docker registry.
Rules:

• Container must be paused during the commit process. Technically it is not impossible to commit active container - but this will cause lots of integrity problems.
• To remove complexity, committing & publishing can be only initiated from a single container session.
• During the execution, both target session and its child (the actual kernel) should fell into a new COMMITTING status.
• After every procedures are completed (even if there is an error on either processes) we need to roll back committed session to stable RUNNING state.
• User should be able to publish committed container only to non-global container registry.
• Before making an RPC call to the agent to perform commit & push, the manager should check the availability of the target container registry (e.g., permissions, quota, etc.) first to report the expected failure earlier.
• Every progress should be reported to API caller. For this objective we can utilize our bgtask library.
• (Optional) Update user/project_resource_policy to limit maximum container size possible to publish. Querying size of files in a container can be possible by looking up SizeRw value of container info (API Reference).