LU-1842 quota: add quota disk operations

LU-1842 quota: add quota disk operations

Add low level quota disk operations that will be used by both the
QMT (quota master) and QSD (quota slave) to manipulate on-disk
structures.

Signed-off-by: Johann Lombardi <johann@whamcloud.com>
Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I783bf9519cf37fdfa35a04cff8207825a7ab1b49
Reviewed-on: http://review.whamcloud.com/4042
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <tappro@whamcloud.com>