LU-10018 protocol: MDT as a statfs proxy

LU-10018 protocol: MDT as a statfs proxy

MDT can act as a proxy for statfs data. this should
make df faster (RTT vs RTT*(#MDTs+1)) and enable
idling connections so that clients don't connect to
each OST just to report statfs data. the protocol
has been changing slightly to let MDT differentiate
self and aggregated statfs.

also, obd_statfs has got a new field "granted" where
OST reports how much space has been granted to the
requesting MDT so that space can be added to available
space.

client's NID is used to distribute MDS_STATFS among
MDTS.

Change-Id: I59e03cb5abf809ae8820f874ec51dd2b74e1806c
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: https://review.whamcloud.com/29136
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>