LU-827 lov: Implement a per file data_version.

LU-827 lov: Implement a per file data_version.

This patch extends getattr RPC to also send in its reply a data
version for the file content.  This only tracks data object changes.
This does not track chown/chmod on OST object.  This value is intended
to be only compared for equality.  A new call in liblustreapi is
available and through 'lfs data_version'.

Signed-off-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Signed-off-by: Johann Lombardi <johann@whamcloud.com>
Change-Id: I5d748e096b11627c05f86ef6f3f6906bce120893
Reviewed-on: http://review.whamcloud.com/1671
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>