EX-7989 utils: lpurge to dump empty slots

EX-7989 utils: lpurge to dump empty slots

empty time slots may still have detailed stats explaining
slot's emptiness: how many objects were scanned, how many
had no pfid, no mirror, etc.

Test-Parameters: trivial testlist=hot-pools
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I7e8f594d8615a5e964c01ec1baf2a2b5d3e9cce3
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51796
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Colin Faber <cfaber@ddn.com>
Reviewed-by: Alexandre Ioffe <aioffe@ddn.com>