Attempt to implement #332 - flattening layers

[vladia]

Please review, it implements basic flattening of layers, suitable for improvements of build process.

Example of flattening, It builds a new temporary image with many layers using Dockerfile and then it creates a single-layer new image that is diff from parent to the temporary image and uses settings from it:

cat Dockerfile |
docker build --rm -t temporary:tag - &&
docker flatten parent:tag temporary:tag parent:newtag &&
docker rmi -f temporary:tag

[ncdc]

You might want to take a look at #4232

[tiborvass]

@vbatts and the other maintainers are currently working on a new image format that will support these advanced operations without destroying history of how the image was built. We can close this as it will be addressed in the new format.

[toddsampson]

@tiborvass Any links to information on the new format yet?

[tiborvass]

@toddsampson There is ongoing work on distribution/distribution#62

Basically the problem is that user data and dependencies (think blob layers for the filesystem) are bundled together.

Separating the two will enable simple squashing of the blob dependencies possibly via API/CLI, without busting the build cache. I wonder whether we should store the history of a squashed layer in some metadata field (cc @jlhawn).