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.

Signed-off-by: Josh Hawn <josh.hawn@docker.com>
graphdriver interface name change, typo fix

Signed-off-by: Josh Hawn <josh.hawn@docker.com>
Merge pull request #7989 from jlhawn/graphdriver_differ_refactor

Refactor use of graphdriver.Differ