LU-3343 mdt: HSM coordinator main thread

LU-3343 mdt: HSM coordinator main thread

This patch implements the HSM coordinator. It is a MDT
thread in charge of schedulling HSM requests to
agents.

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I18fb2fad94b3972b9a09fd093e259f4ad50d810f
Reviewed-on: http://review.whamcloud.com/6912
Tested-by: Hudson
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>