LU-1333 hsm: Add hsm_release feature.

HSM Release is one of the key feature of HSM. To perform HSM
release, clients need to acquire the file lease exclusivelt and
flush dirty cache from clients. A special close REQ will be sent
to the MDT to release the lease and get rid of OST objects.

Signed-off-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I77eeccf81a852955354a382d80b887df00d1c4fc
Reviewed-on: http://review.whamcloud.com/7028
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>