On systems that supports reflinking (i.e. btrfs) this means the dummy
backend is much faster at copying files and will be sharing file data in
a CoW fashion.
On my (btrfs) system this makes "docker run ubuntu echo hello world" go
from about 3 seconds to about 1 second. Not instant, but clearly better.
cp --reflink=auto is availible since coreutils 7.5 (around 2009), so this
seems pretty ok to rely on.
cp is also better at preserving file metadata than tar, so for instance
it will copy xattrs.
This moves Driver.Size() to Differ.DiffSize(), removing the empty
implementations in devmapper and dummy, and renaming the one in aufs.
Then we fall back to a container.Changes() implementation in the non-aufs
case.
This adds a generic Status call in the Driver api and
implements if for the devicemapper backend.
The status is an array of key/value strings rather than a map so that
we can guarantee some static order of the docker info output.
