LU-7340 mdd: changelogs garbage collection

LU-7340 mdd: changelogs garbage collection

When changelogs are almost full (few number of
free entries in catalog), try to recover some space
by unregistering users that are idle since too long,
based on new Changelog User record field (in fact,
using previous cur_padding unused field) to keep
track of last user's changelog cancel request time,
or based on gap between user index and current
ChangeLog record, for older registered user.
sanity/test_160[f,g] have been added to verify feature.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I2100b101772e6d027675e5efa5606d4be24342a0
Reviewed-on: https://review.whamcloud.com/27103
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Stephan Thiell <sthiell@stanford.edu>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>