LU-3446 mdd: changelog index to survive MDT restart

LU-3446 mdd: changelog index to survive MDT restart

If all changelog records are cleared the current changelog index is
initialized to zero when the MDT starts. This confuses applications
that rely on the index to track their position in the changelog
history. To avoid this, initialize the current index from the
registered user with the highest end record index.

Add a sanity.sh regression test. Update changelog test function
test_160() and its helpers according to current style guidelines.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Change-Id: I841bbf84c718bac560b6eea4b18de9918b889a90
Reviewed-on: http://review.whamcloud.com/6642
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>