Investigate incremental pull

[fkorotkov]

A common workflow is when a remote image is just getting updated and re-pushed under the same tag. For example, latest. Then multiple hosts will try to pull the same image when they have latest pointing to the old sha@....

It seems on a pull of a tag that is already presented locally we can try to compare if layers has changed and pull only changed layers.

[edigaryev]

Investigation results:

• Chunking for Tart.ods

PRs:

• tart push-side: tart push: use fixed size chunks to allow for better deduplication #821
• tart pull-side: tart pull: try to re-use local VM image layers to speed-up the pulling #825