LU-10602 llite: add file heat support

LU-10602 llite: add file heat support

File heat is a special attribute fo files/objects which reflects
the access frequency of the files/objects.
File heat is mainly desinged for cache management. Caches like
PCC can use file heat to determine which files to be removed from
the cache or which files to fetch into cache.
This patch adds file heat support on llite level.

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I168fc657f0c859311e5114191b60047646909be0
Reviewed-on: https://review.whamcloud.com/34399
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>