archive.ChagesDir() - faster and better implementation

This replaces the current version with the latest version from the
dm branch. Advantages in this version:

We only scan each directory tree once, which means less i/o and less chance
of container changes inbetween the two scans causing inconsistencies.

We avoid comparing some fields for change detection:

* Inode
* size-in-blocks
  These can change during a copy operation (e.g. in the dummy backend)
  without needing to actually reflect a change in content or metadata.

* Ctime
  Any copy operation will create a new Ctime value, and there is no
  API to change it to the "source" value.

* size for directories
  The size of a directory doesn't have to be the same just because you
  recreated the same content as another director. Internal details in
  the filesystem may make these different with no "real" change.
Merge branch 'dm-plugin-new-ChangesDirs' of https://github.com/alexlarsson/docker into alexlarsson-dm-plugin-new-ChangesDirs

Conflicts:
	archive/changes.go