Refactor use of graphdriver.Differ

Some graphdrivers are Differs and type assertions are made
in various places throughout the project. Differ offers some
convenience in generating/applying diffs of filesystem layers
but for most graphdrivers another code path is taken.

This patch brings all of the logic related to filesystem
diffs in one place, and simplifies the implementation of some
common types like Image, Daemon, and Container.
Oh I think I understand now, you meant for documentation? If the abstract driver is not visible they won't know what a Driver is since it embeds it. Hmmm... so I guess we can leave it exported :)