LU-8475 target: use slab allocation

LU-8475 target: use slab allocation

The patch adds kmem slabs for target threads and session info
to improve allocation and better accounting.

Signed-off-by: Alexander Boyko <alexander.boyko@seagate.com>
Signed-off-by: Alexey Lyashkov <alexey.lyashkov@seagate.com>
Change-Id: Ia0a93d410618c5d7724f2dcc86f1bcb9ae32e572
Seagate-bug-id: MRP-2836
Reviewed-on: https://review.whamcloud.com/21654
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>