LU-3340 mdt: HSM memory requests management

HSM coordinator keeps an in memory list of running
requests send to the agents. This list keep track
of the progress of the data movement.
This patch implements the helpers used to create
add remove update component of this list.

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: I6b68a1342ad485792037607ce84039a0b085d72e
Reviewed-on: http://review.whamcloud.com/6530
Tested-by: Hudson
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>