LU-2424 ptlrpc: add rpc_cache

Add rpc_cache for allocating ptlrpc_requests.

Xyratex-bug-id: MRP-689
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I9e442f8309d19b3320727c3fbfcd7de371032369
Reviewed-on: http://review.whamcloud.com/6874
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>