EX-3142 tests: changelog processing verification

EX-3142 tests: changelog processing verification

add extra counter to lamigo stats to catch gaps in changelog
processing. add a new test (hot-pools/60) to verify that no
gaps happen (i.e. lamigo gets all changelog records), verify
that the changelog is purged properly.

Test-Parameters: trivial testlist=hot-pools mdscount=2 mdtcount=4
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I34d9d6f6f7f5766d945df43ae7d43dab7c70cef1
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/48434
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>