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.
If `graphdriver.ContainerDiffSize()` panics we'll have much bigger problems then one locked containers :) In go you shouldn't assume that your code panics for some reason or you'll be doomed to `recover()` in 90% of code. I suggest just `container.Unlock()` after `graphdriver.ContainerDiffSize()` without closure.