LU-4640 mdt: implement Remove Archive on Last Unlink policy

LU-4640 mdt: implement Remove Archive on Last Unlink policy

This patch introduces RAoLU policy where an implicit remove
request will be sent to archive/Agent upon last close of an
unlinked file.
Policy can be enabled/disabled using an lprocfs tunable.
If CDT not running, requests will be queued.

test_26[a-c] related tests have been also added in sanity-hsm.

This patch also contains a small fix to prevent unnecessary
progress infos to be gathered for REMOVE requests.

Patch now also handles cases where unlinked file is closed
from mdt_export_cleanup() after Client eviction. And
specific test_26d has been added in sanity-hsm.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I7affb20b2834bcd0618412349fc3adc7f6744de0
Reviewed-on: http://review.whamcloud.com/14384
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>