LU-3531 llite: move dir cache to MDC layer

LU-3531 llite: move dir cache to MDC layer

Move directory entries cache from llite to MDC, so client
side dir stripe will use independent hash function(in LMV),
which does not need to be tightly coupled with the backend
storage dir-entry hash function. With striped directory, it
will be 2-tier hash, LMV calculate hash value according to the
name and hash-type in layout, then each MDT will store these
entry in disk by its own hash.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I14bb6bd81aad6fd59dcc22cf4bcea9d341dca2a1
Reviewed-on: http://review.whamcloud.com/7043
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>