LU-1710 lvb: variable sized LVB support

LU-1710 lvb: variable sized LVB support

Originally, the LVB on wire is fixed sized only for transferring
object {m/a/c}time and block/size information.

But with introducing layout lock, new quota and nanosecond features
in the near futures, such fixed sized on wire LVB structure cannot
match the requirements from these new features.

So change on wire protocol to allow variable sized LVB transerred
between nodes.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ifcd2d303495d2b3d1c09c38af823d246fff196fa
Reviewed-on: http://review.whamcloud.com/3965
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>