LU-1842 quota: quota identifier unification

LU-1842 quota: quota identifier unification

This patch defines a generic lquota_id structure which is an union
of all the possible identifier types that can be used with quota,
this includes:
- 64-bit user ID
- 64-bit group ID
- a FID which can be used for per-directory quota in the future

Signed-off-by: Johann Lombardi <johann@whamcloud.com>
Change-Id: Idead35f27ee9d07b9b25f39b791a1ad8778e0f2d
Reviewed-on: http://review.whamcloud.com/3888
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>