LU-3270 statahead: use dcache-like interface for sa entry

LU-3270 statahead: use dcache-like interface for sa entry

Rename ll_sa_entry to sa_entry, and manage sa_entry cache with
dcache-like interfaces.

sa_entry is not needed to be refcounted, because only scanner
can free it, so after it's put in stat list, statahead thread
shouldn't access it any longer.

ll_statahead_interpret() doesn't need to take sai refcount,
because statahead thread will wait for all inflight RPC to
finish.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: Ibd55227f4c0710139bd039a4e7232dc1cb5041be
Reviewed-on: http://review.whamcloud.com/9664
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>