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.
Ok.  I'm a little -1 on the name change because no where in the graph drivers do we have a notion of a conatiner or image and they could be used outside the context of docker to do generic layered filesystems for other projects.  I would probably lean towards keeping the names the same.