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.
I would add a comment explaining that this interface exists solely to tell a driver without the Diff methods. It is not intended to be used by client code. I'm still wondering whether it should be exported. If we decide on keeping it, we would have code duplication of this interface.