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.