LU-847 quota: client retrieve quota usage directly

LU-847 quota: client retrieve quota usage directly

Current 'lfs quota' sends getquota RPC to MDS, and MDS is responsible
for retrieving disk usage from all targets, this scheme will be
changed to client retrieving disk usage from all targets directly.

This patch addresses the compatibility issue as well: If the getquota
returned by MDS has QIF_SPACE, client just trust the disk usage
returned by MDS, otherwise, client has to issue RPCs to collect disk
usage by itself.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Signed-off-by: Johann Lombardi <johann@whamcloud.com>
Change-Id: I35526c453033809d2f23b1806b4783f5011e3fa2
Reviewed-on: http://review.whamcloud.com/1570
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>