Refactor to optimize storage driver ApplyDiff()

To avoid an expensive call to archive.ChangesDirs() which walks two directory
trees and compares every entry, archive.ApplyLayer() has been extended to
also return the size of the layer changes.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #9720 from jlhawn/apply_diff_optimize

Refactor to optimize storage driver ApplyDiff()