LU-925 agl: trigger async glimpse lock when statahead

LU-925 agl: trigger async glimpse lock when statahead

Client will send async glimpse lock RPCs to OSTs for file size
attribute before stat files to accelerate traversing large dir.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I9bf850abdd3c02c8470ddfcd91a3dc3ef7819c6d
Reviewed-on: http://review.whamcloud.com/1692
Reviewed-by: Lai Siyao <laisiyao@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>